It's easier to make world breaking changes when the world is really small. If git could magically just get everything and everyone to cut over and use git 3.0 in a magic instant, it wouldn't be having this problem.
Here's a stoichiometric bird for you:
:%s/git 3\.0/python 3.0/g
It seems Fossil can use multiple hash algorithm within the same repository. Yes, that decreases overall security since the hashes pointing to older objects can still be spoofed.