Well, we're a bit more nuanced about backward compatibility than that :)

I doubt that people *intentionally* rely on the behavior, true, so a change could conceivably be made in a new release.

At this point you need to take your arguments to python-ideas or python-dev (probably the latter), where the people who built this PEP and its implementation can address your concerns better than I can.  I could be wrong, since I'm just a user of generators, not one of the implementors.