For what it is worth - I am one of those users interested in getting curved tracks in v6 and am actively working on improving KiCad’s support of it because it is important for me.
I consider it quite a basic feature and it would be difficult to convince my colleagues to switch to KiCad if this wasn’t supported - almost all of the designs in my company have at least one track that is curved. The CADSTAR importer would have had to approximate curved tracks with piecewise linear segments which would make them uneditable.
Also, RF and flex boards are not the only reasons for curved tracks: ease of layout in circular/round shaped PCBs is another that comes to mind as well as possibly increasing routing density in some scenarios.
I am not sure what “basic” feature you are missing, but if it is really important for you can develop it yourself or pay a developer to develop it for you to meet your timeline. KiCad Services have very competitive pricing for these requests - I can thoroughly recommend them.