Which brings me to another point: not all breaking changes are equal. Sometimes, a change to a syntactic element is needed. But if there's an identical replacement for it, and it can be updated mechanically, there shouldn't be a problem (in theory).

Making constructions or the standard library do things (subtly) differently without a fall-back to the original functionality is a much bigger problem: then you have to inspect every occurrence and verify if it still works under all conditions.

What Python did is far beyond that; — it knowingly broke what everyone was using with the full knowledge that almost no Python 2 codebase could run under 3 unmodified.