Yeah, but would not they have to create v4, v5 and so forth every N years, for different hash functions?

Sure, but this is not any more expensive than any other versioning scheme you might be thinking of. Consider also that they got 19+ years out of v1, and that there is no reason to believe SHA2 will be broken faster than SHA1.

Probably, but would it be possible to make it so that one could easily swap the hash function? Like I am curious about the details here. I think it would be. Clients probably will have to implement a couple of commonly used hash functions, and so forth. I am not sure how it would work in practice or if it is worth it at all. I am interested in all the details though.