Proof that matters to whom? Looking inside any modern AVR tells me I don't like their product design strategy. There are AVRs that might measure well and might be reliable, but I wouldn't have one if given to me gratis. It's just a preference, but based on some reasoning beyond I don't like their looks.
Matters to people who care to know.
Well, if you must have separates, then that is the only reason that matters to you. It doesn't even matter if the AVR measures better and have better Subwoofer EQ and Dynamic EQ, etc.
But to other people who want to know if separates really have better sound quality and better reliability, it matters to them. It may matter to them if something like the Denon X4000 has Audyssey XT32 + dual Sub EQ + Dynamic EQ and the separate components do NOT.
With my experience, there is no way I would ever go back to Direct mode for music or movies because Audyssey XT Sub EQ + DEQ improves the sound quality so significantly like night and day. I suppose many people may never experience the same thing I experience and most people will never even try. I guess it will never matter to them. But it matters to some people.
Features like AirPlay and iOS remote app also matter to some people. I stream lossless music files from my iPad to my Denon all the time. And use the Denon remote on my iPad exclusively. Of course, it may not matter to everyone. But for some people it matters a lot.
And as far as taking a brand new electrical component gratis, I would take anything.