I see your point. But so far we didn't hear many complaints about this feature from the users. The main problem is in forgetting to version code that changed. But the system has protections against such cases.

It doesn't end up in spaghetti code as old branches are proactively removed after they are not needed anymore. Temporal provides APIs that allow to count number of workflows using each version.

I don't think it is a leaky abstraction as it is indeed a part of the business logic of deciding how old version of the state should become a new one. I don't think there is a generic solution that allows implicitly migrate old state to a new state on any long running computation.