I think there is an implication that if you buy a game which is online by nature (e.g. an MMO) that the servers can and will shut down eventually. My cupboard is filled with defunct MMOs. And people do not “own” any commercial software per se, they run it under licence.

So I don’t see that Ubisoft has any legal obligation here. But as a good will gesture they really should put the server code in escrow, or open source chunks of it so that games can continue to enjoy life after the company itself has no economic incentive to continue running it.