but you can also never tell for sure whether you've reached it
Sure you can. Keep raising the bar until you're observing unavoidable failures. Theoretically, everyone will at some point, with the delightful side effect that you might achieve some phenomenal results from the interim exercise!
But how, in practice, do you keep raising the bar? What is "the bar" (i.e., the measure of your intelligence) to begin with? If you mean "keep learning new things until I no longer can," that doesn't sound like a recipe for unavoidable failure so much as a practical life plan for using your mind.
And when you do encounter failure, how do you know for sure that it's unavoidable? I've had some wonderful experiences where, having thought I'd hit a wall, I discovered that the scope of what I was learning was much larger than I thought after I discovered key new pieces of information. I've also been able to improve my overall thinking ability through new areas of research and intellectual practice, but I don't think that's making me "more intelligent" (i.e., raising my upper limit) so much as showing me how to think at that level, which would always have been available to me if I'd known it was there and how to access it. Any of us could have a similar expeirence that we misinterpret as "unavoidable failure." In practice it doesn't seem possible to tell for sure which is which.
