Sure, everybody using UTC when they actually want TAI would be a no-op, but then you irreversibly break everybody who actually wants UTC and assumed that UTC would not change meanings.

The people who would be unaffected by the redefinition can already just trivially switch manually (as we already assumed that just redefining things under them would work), leaving the UTC people alone. There is no good reason to silently break all programs carefully designed to use UTC correctly to fix all of the programs haphazardly written by people who did not know what they were doing and used UTC when they actually wanted TAI. Especially since fixing the wrong use of UTC is so trivial that we assume it can be done with no modification.