On the other hand, within 120 years (and sometimes decades earlier), every human so far also has reached reach a point where they can't do anything anymore, reliably or otherwise. I don't think humans provide much of a precedent of sustained growth in reliable over anything other than a fixed period, and it's not clear to me that we have any idea whether or not we'll reach a ceiling in how reliable we can make LLMs before they outperform humans in tasks like programming.

I'm not sure that's generally true. However, older people have a track record, and a reliable older person is likely to be more reliable than a younger person without such a track record.

Reliable has different meanings. I think in this case the meaning is closer to "deterministic" and "follows instructions." An older worker will more reliably behave the same way twice, and more reliably follow the same set of instructions they've been following throughout their career.