I’ve been pondering this for a few years now too. My desktop is a late a late 09 Mac Mini and Apple is in the process of obsoleting it now. I spent a few days last year looking into installing Linux on it but Apple had made that difficult, if not impossible, which really just pushes me further towards Linux fulltime.
The truth is, a Raspberry Pi 3 is pretty darn close to being usable right now. Geanie is a capable replacement for BBEdit and comes pre-installed on Raspbian now and the latest release is really quite polished.
It may be a few more years for me (at most) but when this Mac is no longer able to keep up my next desktop will be Linux and I’ll be happy about that. It will be interesting to read about your experiences with this. Looking forward to it.