The difference is that this breakage would be immediately visible. All code that mishandles these hash would immediately break. With collisions it can remain undetected for a long time, and potentially until somebody smart and with bad intentions finds a way to break your system in some creative way.

And again, if we had done this when it should have been done, i.e. pre-2010, we wouldn't be having this discussion. The longer we wait the more painful the migration will be whenever somebody manages to actually bruteforce collisions for git commits. We're not there. Yet.