For sure, I have no problem believing that with a perfectly set up pair of 8Ks in a good room you can likely get just as good reproduction for music (if you're not interested in Trifield, etc.). However, I would argue there is still a place for, e.g., properly integrated subs and RC in most real world domestic listening rooms.
Usually, speaker placement is a compromise between best imaging/soundstage and the need to flatten out bass response as much as possible. When speakers are truly full range so both are in one box so to speak, the best of both isn't always achievable. This is the case in my room for example.
Properly set up subs can be placed where you wouldn't want to place your main L/R pair to minimise peaks and nulls in the very low bass, and RC can tame the last bit. Sub 80Hz, this is notoriously difficult to achieve with room treatments, as they would have to be huge and probably domestically unacceptable.
Another example of compromised placement for 2-channel also occurs in my room, due to the constraints of that space and the cinema screen. We have to sit a little closer to the speakers than the distance the speakers are apart so they don't get in the way of the projector. This means there is a perceived 'hole' in the stereo image, something a well-integrated centre would mitigate for music (oddly enough it's not an issue for movies where I suspect the image psychoacoustically 'locks' the sound to the screen).
I've come to think of a processor as a little like an audio Swiss army knife for solving real world problems, as well as a vehicle for multichannel soundtracks, etc. Hence the desire for one which sounds as good in basic SQ as my 818v3 does in 2-channel - I would very much like to have my cake and eat it!
