I doubt AI will ever generate straight up machine code directly as a best practice. Hierarchical, high-level abstractions help even AI reasoning. Re-reading some undifferentiated machine code make it harder to reason about even for the AI, while having a compact representation expressing the high level algo is much more efficient.

Regarding seniors of tomorrow, I think there are big differences between good devs and sub-mediocre ones, and I think their proportion won't change, and the good ones will have the drive to use the AI to learn and understand because they are simply curious and want to know. But most programmers don't care at all, and that's the case even today, and they will be less forced to learn. But I don't think we will lose much with this. Important things are held up by a small proportion of engineers, a Pareto-like principle.