I don't expect them to, but I don't not expect them to, if that makes sense -- it feels like an "all bets are off" kind of thing to me. The industrial design of the iMac is likely to finally change when it goes ARM, and it doesn't strike me as impossible that they'll make it slightly more open, or at least not make it worse.

On the one hand, Apple loves to eliminate options and lock things down, both for quasi-defensible reasons like simplifying product lines and for less defensible ones like increasing profit margins (and making everything obsessively thinner, like they're in the grip of some industrial design anorexia). On the other, their most recent hardware design changes have often shown response to customer complaints (e.g., replacing the butterfly keyboard with an improved iteration of the "Magic" keyboard) -- and, if they really intended to lock down the Mac like iOS, the move to ARM and the sweeping UX changes in macOS Big Sur would almost certainly have been when that happened. The fact that it hadn't happened makes me considerably more skeptical it's going to. (I'm also more skeptical now that iOS will ever be allowed to blossom into a full general-purpose OS, but that's a different topic.)

As an aside, I'm not sure whether adding the T2 chip would make it more complicated to use a third-party SSD. It's my understanding they function as the SSD controller and do some kind of wonky things, but I am not taking the time to look that up and could be completely wrong. :)

I feel like they are taking a page out of consoles, at least if the iPad's influence is to be felt on future Macs. Relatively rigidly defined hardware specs and the apps are designed to take that into account.

The difficulty of lugging a 27" machine around is probably the only reason they make it user upgradable.

but SSD is soldered on motherboard though. previous models allowed to change storage..