> designed under the assumption that each update changes everything.

But changes that only affect a few things are rare. If you update glibc you have effectively updated everything because everything depends on it. So everything needs to be retested.

If you install an update for almost any package, that could change the behaviour of any program using that package, which could change the behaviour of any programs that depend on that, recursively. So you essentially need to retest everything for almost every change. and if you need to retest everything anyway, you may as well bunch the updates in larger blocks.

But you say that one tiny bugfix somewhere could not possibly break a system on the other side the world. Except yes it can. It's rare but it happens that (e.g) a tiny optimisation somewhere reveals a race condition somewhere else bringing down an entire system. Bureaucracy doesn't appear out of nowhere, some of those regulations are written in blood.