Vim is expected to continue to work everywhere, even on some bank's circa 1980 mainframe. NeoVIM has become an effort to revitalize the project. Support for ancient systems was tossed almost immediately (eg no more 8.3 filename support code). NeoVIM also included some much improved default configuration values.
Also you probably want ed (the standard text editor) for POSIX compliance. (Fun fact, vi is not required by POSIX, so you will not find it on every single POSIX compliant host.)
Continuity I get, but how much are we collectively burdened by having a few ancient uses determine the wants of the many?
Lots of designs from the past can no longer connect to the internet in a meaningfull capacity but new hardware today might meaningfully last far longer than the developers could even anticipate.
It's not that I actively hate anything that isn't a recent 64-bit Unix, but that the people on other systems are either 1) hobbyists who are competent to port stuff to their own systems, or 2) companies who aren't paying me for extraordinary support.
Bummer if something I wrote last week doesn't run on HP-UX, but it's not going to cost me sleep.
Because of neovim and the power of forking, essentially nothing. The ones who favor stability choose one program, the ones who favor bleeding-edge chooses another that matches.