they don’t have to keep supporting, just not make it unavailable the best case would be if they made it open source, in that case other people could keep maintaining it, but would be against their profit incentives
it used to be the case that when you weren’t able to enforce DRM on a piece of software anymore, you would offer it as a free download so people who bought it wouldn’t lose it
if your game is in production you risk introducing bugs with every update, but you will probably want the latest version of the engine for your new game