Personally, I believe it can be fixed via key rotation (e.g. there are 3 inputs to the PAC algorithm. The pointer, a "modifier", and a "key" e.g. APIAKey_EL1).

I would have added that as a potential mitigation in the mitigations section. I think, say, changing the key every so often would be a reasonable task for a kernel to do, especially in the timeframe that this was exploited (about 3 minutes)