Breaking compatibility is generally avoided to the utmost. Even security-sensitive things like TLS continue to support older, less secure versions to retain compatibility with peers that haven't been upgraded yet, much to the chagrin of everyone when they screw up the version negotiation, but better than the chicken and egg problem where nobody can upgrade until everybody has.

But the other point is that the claimed security improvement doesn't actually seem to be there in this case. They haven't made it so you can't have a program continue to run after the end of the current session, they've only changed what you have to do to make that happen, thereby breaking everything that did it the traditional way.