Why would that be the default assumption?

Because they’ve gotten good enough at lots of other things, and the RL keeps improving them, so enough RL should make them good enough at the focus areas.

I've gotten pretty strong in the gym, my bench has improved to two plates. I see no reason why it won't continue to improve until I can bench my house.

I think it’s a bit closer than that. You can bench two plates, and “good enough” is two plates on the bench and a three plate squat. LLMs haven’t started leg day.