I mean removing undefined behaviour from signed overflow isn't the problem. The problem is that when a programmer previously wrote a signed operation that could overflow, they didn't specify what behaviour they really needed.

You can define a behaviour today that they'll all get, but you can't define a behaviour that matches what each programmer needed when they wrote their operation. Some of them can wrap safely. Some of them would need to saturate. Others would need it to abort.

The problem isn't that the behaviour is undefined per se, it's that you can't tell afterwards which potential behaviour the programmer needs, and choosing one behaviour e.g. wrap will break everyone who actually needed saturating, or trapping, etc.