There is a Pcb designer and a schematic editor and If we use our other PCB works in our current PCB desing, then there is no need for a footprint notion anymore. In another words, “Footprints are rarely changing small circuits”. Is there anything I am missing?
Did you use google translate?
Or he used a bottle of something?
EESchema - draw your schematics with this
Symbol Editor - edit/make your symbols with this (has a browser attached)
PCBnew - draw your layout with this
Footprint Editor - edit/make your footprints with this (has a browser attached)
@ceremcem - which one is superfluous in your workflow again?
I think all the questions from the OP are coming from a manual fabrication process, where PCBs are creating by photo-resist or toner transfer etc.
You can create a mask in any graphic package, like Paint/gimp whatever, then print to paper or transparency. Reusing part of a PCB layout becomes easy - you can use literal copy and paste to create a new bitmap or paper copy.
So ceremcem is trying to duplicate that process in Kicad, which I don’t think is possible. Since the manual process is 20 years old, modern software doesn’t support that type of process.
Possibly the closest is a panelizing script, which takes gerbers from different projects and merges them. I have used gerbmerge for that.One could also do a similar thing with bitmap images, with a bit of scripting.
I wondered about the possibility of a script to convert a PCB into a component, but I think it would take too much intelligence to do that in a script.
Come on, is it really like google translate? At least after those edits? 
Actually, you are right about your guess. We are a company producing rapidly prototyped embedded/server side IoT systems. As a cheap, fast and flexible solution, we use to toner transfer technique to produce PCB and small piece of wires and solder in place of through hole plating. Following PCB is produced in 1.5 hours:
So, I can not care if it is a century old technique or the one used in star wars, we use and need that technique.
But this is not the case. I never fully used a pcb drawing tool before, but I really use and love SketchUp, a 3D CAD program. When you start drawing something (for example a house) you can do it just by using a pencil tool. Then, if you love your window and decided to re-use (or share!) you need to just select it and “make component”. Here someone presents:
He doesn’t manufacture a PCB but he still use window footprint everywhere.
I believe one can “draw” a layout in KiCad by just using lines and polygons on copper layers. No footprints needed 
But for such workflow I’d suggest using an advanced graphic editor that have many more capabilities than KiCad (simply because KiCad is an EDA tool, not a graphic editor).
So, do you got schematics that go with those toner-transfered-sub-circuits or do you do those ‘on-the-fly’ without schematic?
I can imagine a script in KiCAD that would take a block and convert it into a footprint, converting tracks (segment) into graphical lines (fp-lines), reusing the pads, etc… only thing that needs to be taken care of would be the need for tiny pads that go at the ends of fp-lines to be able to lay new tracks to those ‘wires’ of the footprint.
All in all I don’t get the workflow… can’t you reuse a project, and go from there?