The worst case is it doesn't work at all due to protocol changes between the DualSense and PS5 console, but the DS4 working in limited capacity implies that's an unlikely outcome.
I'm going to take a wild guess that the protocol won't change much. I mean it was technically possible to use a DS4 on a PS3 after an update, so they seem invested in the IP.
Nevermind all the games that have invested parties in the ds4 controller architecture. I'm wondering if some of the fighting game producers will support the older controllers in their next gen games.
Going to be tough to get a bunch of those people spending a ton on a new fight stick/fight pads. This isn't a huge segment of the population, but they are console buyers themselves.
There's also a handful of PC games that support DS3/DS4 out of the box, Rocket League comes to mind.