It really doesn't. Either the system is working, or it isn't. People might be using it, but people use all sorts of broken systems.

It is indeed worth weighing the cost of making a breaking change, but given that it happens all the time in all sorts of projects, it's clear that only very few projects subscribe to your notion that breaking users are an absolute sin.

Even Linux has broken userspace programs when fixing bugs. It's only happened a few times, but it happens.