I don't know what the future holds, but I do know that Junior devs are cheaper than senior ones. I could envision many junior devs coding (with the help of AI) and a few senior devs doing mostly code reviews.

Honestly, that seems like a recipe for dysfunction. You have junior devs relying on a crutch and failing to develop their own skills, then you have seniors getting their skills dulled/stagnating because code reviews aren't how you do that either (it's just an limited application of existing skills). Also, code reviews are a drag, and I can't imagine someone staying motivated if that's literally their whole job.