Exactly: and that was a wrong decision for anybody but the developers of Python.

Everybody else prefers having something that works: "The improvements are welcome, but please allow us to to run our old programs too, thank you, and allow us to use that new feature only once we need it."

It's an obvious expectation. We would also hate a new version of a word processor which wouldn't open our old documents. Or a new version of Photoshop which wouldn't open our old pictures. Or a new version of the browser where only the newest web pages are visible.

It follows that it was absolutely technically possible to have a new version of Python in which the old programs still work. It's the failure of the developers that they haven't made it.

Compare that decision of theirs with the policy of Linus Torvalds who insists that the old user programs should never break on newer kernels.