I'm not an expert here but I have to say this feels like a very weak objection.

p points to P1. One thread reads through p. Another thread races with that and mutates p to point to P2. The result is the first thread reads from either P1 or P2 (but no other object).

This seems totally fine and expected to me? If there's a data race on a pointer, you might read one or the other values, but not garbage and not out of bounds. I mean, if it could guarantee a panic that's nice, but that's a bonus, not required for safety.