I don't agree that SHA-1 is much longer feasible for git.

But I also don't think that switching to SHA-256 must be painful. A git2->git3 converted repo could just store all the past hashes, so existing links don't break.

I was thinking the same. Can't the git CLI see a hash and say "well, I don't see any matching SHA-256 hash, but let me check the Legacy SHA1 hashes I have stored", and still resolve an old SHA1 hash to the correct commit?