Any portable device as a Matter hub would be pretty useless.
The Apple TV and Homepod mini already support Matter.
The Apple TV and Homepod mini already support Matter.
An old iPad on a stand on the table in the breakfast nook is conceptually fine as a Matter controller until someone picks it up and leaves with it, or unplugs it and lets it run out of battery.
Apple forbids this kind of use because it defiles the majesty of their garden and its walls (or something), but that's just Apple doing Apple things.
Expectation of usage. An iPad designer assumes it will be taken all around, leave the house, put in a book bag, etc.
While an old iPad being used as an entertainment management surface won’t, that’s not the prime use case intended for iPad.
Apple decided to focus on products whose primary purpose is a stationary device, like a speaker or a TV-attached device. Sure, you can move them place to place, but most won’t.
And the usefulness (or lack thereof) of the functionality can never be tested by real people, because at Apple: Design is everything.
If the only problem here is "But it could be moved!" then solve that problem.
(Of course, that is not the only problem. We can't get to that point to begin with because Apple is Apple.)
Even if the iPad recognizes it's always plugged in and discharges from 100 to 80 occasionally, it's still a use case that increases the chance of battery swelling.
I don't mean this as "Apple knows best, psh, silly customer" but a bit of "that's one reason this exists" and a bit of being annoyed that you can't run tablets battery-less off AC to prevent it.
It would pretty obviously be easy to add a wizard that prepares the iPad to be repurposed as a stationary home hub, setting the battery to be held at 50% and neither charged nor discharged, sending a notification to your phone if it's unplugged, choosing a slideshow to double as a digital frame, etc. But to be fair to Apple, no other companies would build such a thing either, it's just the consumption oriented system we live in.