It seems pretty bizarre to me to make an argument along the lines of "it would suck to be immortal, because your brain would still age and you wouldn't be able to adapt and learn new skills." I thought the very definition of immortal is that your body
doesn't age, brain included. If we're assuming your body remains young indefinitely, it seems to follow that your brain would also remain in that 25ish-year-old state where it can easily learn new skills, since the loss of that plasticity is exactly one of those aging effects that any form of immortality would have to cure in order to
be immortality. I can imagine forms of immortality that would be a curse, but they all seem to require some kind of special rule that defies logic, i.e. you have 'x' but you don't have 'y' which would logically follow from 'x'.
Granted, even if your brain doesn't lose any of its capabilities with time, you still might run into a limit on the knowledge you can retain/skills you can learn - I'd hope our brains are capable of adapting and forgetting unused skills/knowledge in that case, but the 'hardwired' argument is certainly plausible. Even if it's true though, I just can't see it being that big of a problem - if we had a way to prevent the entire body from aging (not entirely implausible, literally there are
scientists working on it right now) then fixing any side effects this had on the brain's ability to adapt to excessive information/skill storage would be equally plausible. Of course it's all science fiction/fantasy for now, I'm just saying by the time we can stop the body from aging, we can probably do something about the brain running into a capacity limit too.
Of course that's all just physical capabilities, emotionally speaking there's always the possibility that people would run into the "tired of life" syndrome like Tolkien's elves. But I see that as less of a 'curse' that naturally afflicts an immortal, and more of a personal issue that would trouble some (there are people that get tired of life in the mere 75 years we already have!), but not necessarily everyone.