Sorry for not answering your question, but this has been discussed many, many times over. The question from us to you is “why”. Why do you want to use the metric system? The unit used in the symbols and the schematics doesn’t correspond to anything concrete in the real world. Even in printed paper you won’t need anything exact, and if you feel you do, you are using something for wrong purpose. Therefore the default units haven’t been changed in KiCad, and you could as well imagine that all the length values there are abstract unitless values, or some random units like bananabox or µnmi or kicad_sch_unit. Changing the default is useless, fruitless and purposeless for anything else than your aesthetic feeling or your inner engineer obsession.
Actually, you are right. If it’s that hard to make Kicad metric i will use it in imperial. But it hurt for my european brains, and it’s little bit easier to work with something similar.
It is easy to make KiCad metric for the PCB editor, where units matter in the real world. It is hard in the schematic editor, where units don’t matter but it is critical that all pins and wires align. It is best to pretend that there are no units in the schematic editor as eelik said, and do not try to change the units.
In any case, I just checked the code, and while the internal unit seems to be metric (nanometer based), some stuff like the settings dialog you showed us is hardcoded to convert the preferences to mils. So working in metric at the moment isn’t really feasible at the moment (and would be a huge amount of useless work as already mentioned).
I know it’s annoying to work with imperial units if you’re European, but in the case of the schematics editor, you sort of have to live with it.
Life kind of s***s …
When in 2017 I moved from Protel to KiCad I changed my PCBs from everything in mils to everything in mm. But I never even thought to use mm at schematic.
C’mon now, say what you realy feel 
Symbols and Schematics are neither Metric nor Imperial. Everything is ratio based on the grid. The grid just happens to have the “mils” suffix attached to the number.
If you wish to view a symbol or schematic on your screen in Metric or Imperial the zoom needs to be altered. Likewise printing, alter the %.
To avoid serious brain damage, it is best to read “mils” as an abbreviation of " mil limeter s" rather than thousandths of an inch 
There is no way a few lines and squiggles on a piece of paper actually do anything. They are just an abstract guide to help create the real-world, does everything, and MUST have units, PCB.
I don’t like things imperial, except for stout.
I can’t understand all the fuss. With electronics it is mostly decimal Imperial and Metric
The distance between pins 1 & 8 of a DIP 16 package is .1 X 7 in imperial. In metric it is 2.54 X7.
Which is easier to calculate?
It could be much worse. Our forbearers could have started semi conductor packages in 1/64th inches or used British Association Metric which starts as 6mm = 1 and every incremental size smaller is 9/10 the previous size correct to two decimal places. Everyone would then have something to really scream about. 
I am wondering would it make any harm if there change of grid scale value would not be allowed in future version of schematic editor? At least for me it has happened with Kicad 5, 6 and 7 versions that I have somehow (I think by accident) ended up of having a following types of problems.
- I have symbols A and B in schematic editor
- I have drawn connection between pins on symbol A and B
- Somehow I manage to get the grid size value changed for example from 0.025 inch to 0.0100 inch. (Not sure whether I press by accident some shortcut that changes it or do I somehow change between impereal and metric units)
- If I after that click Symbol A, it will move a slightly to align with the currently selected grid coordinates
- Now the connections from Symbol B get’s all disconnected from symbols from A.
Sometimes it’s even impossible to get them connected, until I make sure that all symbols are aligned to this new grid coordinates. → In reality I need to draw lot of connections again as visually they look to be connected but Electric rule checker reveals that they are disconnected.
- Check to make sure your grid is 50 mil.
- Create a selection including your whole schematic.
- Right mouse click
- Choose “Align elements to Grid”
This will re-align everything back on grid.
A really common problem is miss-keying the “M” move key and hitting the “N” key instead.
The N hotkey is set to “Switch to previous grid” which causes problems if you do not notice.
The current grid you are working on always shows at the centre right at the very bottom of your screen. It pays to look at that occasionally.
I solved the N key problem by changing the action of the N hotkey.
There are some major grid changes in Kicad 8 to help solve this problem
I think this was the infamous N hotkey. I think in v8, it’s been made harder to change the grid accidentally, I forget the details but you can search for it.
But this problem has not been changed in Kicad 8.
Didn’t you write this Change the default for Hotkey N in 7.99 which says that in 7.99 it displays the choices for a short while?
Then there was this topic: Shortcut "n" to change the Schematic Grid is horrible! - #60 by JeffYoung which resulted in: Off-grid schematics are too easy to create (#14756) · Issues · KiCad / KiCad Source Code / kicad · GitLab
Why do I have to do your searching for you?
I don’t understand the remark. Who is searching and for what?
All I commented on was the default for the hotkey N had not been changed.
The hotkey has not changed for v8, but v8 uses different grids for editing connecting things like symbols and wires and non-connecting things like random texts.
Pressing n while placing a wire/symbol only toggles between the available options for the connection grid, which I think by default are 50 and 25 mils, so there’s not much danger there.
I agree with your comment; completely: but: I am still surprised that the “powers that be” have retained N as the default for “Switch to next Grid”.
My story:
I only use symbols from personal libraries. Some are “home made”, many are copies from Kicad. Consequently, I have added many more grids to the Symbol Editor, and, because I can, to the Schematic Editor also.
I am aware that because of my additional grids, I must continually check my current grid displayed at the bottom of the screen, because the N key no longer toggles, it scrolls. This is a nuisance for me as I sometimes miss hit the M key while working.
I solved this problem by re-assigning hotkeys. This is not difficult.
Switch to Previous Grid is Shift + N, so I made Switch to Next Grid Alt + N and made Edit Grids N.
Making these changes meant changing grids was now accomplished only with a deliberate action.
Accidentally hitting the N key by mistake now harmlessly shows a grid preference window, so I know I have miss-hit the M key and hitting the Esc key is a lot easier and requires far less thought and observance than wondering why the drawings have changed, what caused the problem and how is that problem fixed; even if it is just toggling between two assigned grids (25 & 50 mil).
I am just so surprised that management, after all the Grid changes to make life easier, especially for newbees, that using the N hotkey to change grids, is still retained.
Thank you very much, I did not know about this solution earlier. Yesterday I had managed to get with one project about 130 warnings again like in the attached picture and was moving parts and lines once again one by one until I remember to check new answers from this discussion.
Not sure whether the problem was the “N” key, but my grid size was 10. I changed it now to 50 manually
and did the steps you suggested and the problem was solved.
Generally speaking, I agree with this sentiment. But if you need to print schematics conforming to a certain ISO/ANSI standard, there are some dimensions that should be exact.
This probably fits in your category of “wrong purpose”, but I have needed to put some simple schematics directly onto the silkscreen of a PCB, and in that weird case, it was important that the schematics had real-world dimensions.
I am American and old, and while I use metric as much as possible, I’m also very comfortable with working in mils, and have been using imperial grids for schematic tools since 1986. But I can see how this could be awkward for our European and Asian colleagues. But I can also see what a logistical nightmare it would be to generate and maintain two sets of symbol libraries, say one used on 100 mil grids and another based on 2.5 mm (not 2.54) grids.