Author here. I don’t think ASCII is the right comparison. True, it would be really hard for anything to compete with Git because a lot of infrastructures we have are already deeply integrated with Git. But think about x86 vs. ARM and how AI might change our ways of producing code.
UTF-8
That validates gp's point though: UTF-8 doesn't replace ASCII, it extends it. All valid ASCII text remains valid UTF-8 while retaining the same meaning. With the momentum behind git it will be hard for something incompatible replace it, but an extended git could catch on.