You say this
"I think this is a correct statement of the industry view: that stereo audio and multichannel AV products are unrelated, and which forms of processing are available, where they go in the box set, and how they are linked together are separate decisions.
I also think times have changed, this is no longer a correct view, if it ever was, and the right approach going forward would be to make stereo boxes that are effectively exactly the same as the multichannel ones except audio n=2, video n=0 — including using e.g., HDMI interfacing and just ignoring surplus channels!"
... but multi channel amps are not know to have got it right for music when compared to hi fi. 9 times out of 10 (completely made up statistic to prove a point) musical sound quality wise; traditionaly a seperate transport, dac, pre amp & power amp have got it right in more ways than cd players & integrateds and cd players & integrateds have got it right in more ways than av kit or all in one systems. So why should the industry change?
Traditionally designing a power source dedicated to one piece of the chain and isolating and spacing those components in their own box works better most of the time. You are right it is not rocket science but science does dictate that this design leads to fewer complications, fewer opportunities for interference and more opportunity for components of larger sizes and larger spacing/circuit shapes.
Bunging it all into one box with the focus changing to making it all fit and then designing a jack of all trades power source and doing the best you can with isolating components is not really hi fi. It is however something to shove under the tv and if the goings on inside the box and output were of no concern then it would be preffered.