The focus for the RPi4's is AFAICT on:
3- The rest
3D is a slowly dying breed, more and more TVs are not 3D capable.
You may have to use the 3D+ quite a bit longer the way I see it.
For TVs I agree totally it likes a puppet theatre on it - even on 55" - but on a 110" cinema screen it is great. And I think I'm not alone having a huge number of 3D movies and like to watch again
BUT another question (I'm not a programmer) -
wouldn't be possible, if Kodi running on a Pi4 and detects a MVC 3D File, to redirect it to a Pi3+ (running in parallel) and "restream" the file?