Frequent updates

The thing is that these kinds of breaking changes in Setuptools can break thousands of packages while still only impacting a very small percentage of packages. Both because of how big PyPI is, and because of ecosystem effects (when your package breaks, so do your dependents, transitively). If Setuptools bundled together multiple changes on the order of the one in version 78.0.0, it would IMHO be pretty unlikely that any given package had to deal with multiple of them at once. (Although the kinds of projects that are destined to be hit by multiple such changes eventually, might be better off just switching to a different build backend now.)

Also, if Setuptools had a slower release cadence for major versions, packages could think more seriously about setting a version upper-bound on the Setuptools version used to build them - since they could re-evaluate the decision less frequently.