Chummer stopped working in XP because there was functionality I needed from .Net 4.5 that I didn't want to reimplement or work around in .Net 3.5; the fact that it was a deprecated OS was largely incidental. At present I have no reason to assume I'd need to abandon support for Windows 7; the next logical step would be to .Net Core 3.0 and a cross-platform UI stack, which is fully supported by all modern operating systems.
As far as future development work goes, I have no interest in working on 6E from the games I've watched and played of it so far. Stefan Pelle's Genesis seems to be fine for that purpose so far.
I'll keep working on it as long as I find it fun and I'm able to do it; if I ever run out of issues that need to be worked on, I'll likely turn my attention towards integrating 4e into the codebase to support both editions and generate more work for myself.