The point is we don't really know what limits exist beyond our small pinhole of perception. During industrialization I'm sure people said the same thing: "What's the point of hand-made clothing if machines can do it?". Today both hand-made and factory-made clothing have their place in the market.
Throughout history, tools have disrupted humanity and we've adjusted and expanded our horizons. To me this feels like another wave, and If I could bet and could collect winnings centuries in the future, I'd say this isn't even the last one.
Some track and field records set in the '80s still stand, e.g., Marita Koch's 400 m, Jürgen Schult's discus throw, Galina Chistyakova's long jump.
Maybe he's lost motivation, but this retirement is not a surprise.
The impression I got from Lee Se-dol in the Netflix documentary was that he had a lot of his identity tied up in this (particularly being the best Go player in the world). Not a huge surprise given the time and effort required to do it, but there's probably a healthier mental framing (I think Kasparov has a healthier one).
If people give up on learning or doing something every time someone or something else can do it better then there's going to be a lot of disappointment as things continue to move forward.