Years ago I called all-in-ones the modern music centre. Justified?
Yes, I reckon that's quite an apt description, but with the emphasis on the word 'modern'.
Many old music centres, although not all terrible, were a quality compromise compared to separates.
Modern all-in-ones are likely to be using many of the same class D amp / Bluetooth etc. modules used in separates (just without so much empty space around them).
They should (hopefully) be designed well enough to minimise the disadvantage of a common power supply too.
They're certainly a tidier, more compact solution. But I can understand why people don't want all their eggs in one basket.
With separates you can upgrade each part if / when necessary.
Then there's the potential cost of repair.
There's no real reason why it should cost more to have an all-in-one repaired.....but.....if it cost 5+ times? the price of a separate, then the repair charge is likely to be significantly higher - just because it can be.
People will naturally be more reluctant to scrap a multi-thousand pound product, than they would a multi-hundred pound separate.