But recursive self-improvement isn't always a recipe for FOOMing; diminishing returns can lead to an asymptotic bound, Zeno-style, even if technical refinements remain theoretically possible.

Yes. Early wins are very cheering and promising, but pretty much all growth curves follow a sigmoid. See the start? That's exponential. See the end? That's diminishing returns. This is one of those curves you see over and over.

So the trick is to find a new sigmoid to climb :-D