I call bullshit! And it's
not because of the teleological implication you're making - it's because you're overfitting a pattern - limited existence - where you have no data to observe the correctness of your conclusions!
You think you see reasons why an overly long-lived human would encounter difficulties because of limitations you perceive that they'll encounter, but your observation set is so limited that your conjectures must be treated as conjectures, not as solid understanding!
The limitations on learning we observe in older human could easily be due to a huge number of factors, almost all of which are extremely solvable. The one mentioned here, that crystalized intelligence, which is an advantage in some ways becomes a liability in this case - that one is not immediately solvable in my view -
But when I discuss radically increased lifespans for humans, and I discuss it often because I want to see it happen and am already participating in a startup whose success I believe will hasten that effort, I often hear people giving reasons much like yours are reasons why my efforts are not worthwhile. Honestly, seriously, people are using justifications like yours to avoid investing in efforts like mine, and it's maddening to hear such an unsure, ephemeral hypothesis - and I honestly regard the conjectures you've made above to be among the most uncertain hypotheses in history of humankind - to be used as a reason to deprive concrete efforts of concrete investment.
Absolutely maddening, especially because I don't even necessarily disagree, but if the actual pattern for humans is that after, say, 1,000 or 100,000 years, we've got too much cruft as a part of us to continue, we'll never know unless we investigate it, and we can't investigate it if everyone just blindly trusts in silly design pattern hypotheses!!!