The example you say about the fiat currencies eventually tumbling to zero can be flipped on Bitcoin the exact same way. If Bitcoin were to reach 10,000 a coin, you assume that it would remain steady and that no one should convert to fiat, but realistically you can't be precise if the price wouldn't crash. When we were at 1,000+ a coin, many speculated we would be "to the moon" a year from that, and the direct opposite happened. I don't blame anyone for wanting to convert to fiat at 10k per coin. It's taking the safe way out (believe it or not) which isn't a bad thing.
People don't have to cash them out/ exchange for bitcoin. They could just buy things with it instead, but I guess there's only so much stuff people can buy or want. I think if bitcoin does reach 1k again a lot of people will just cash them out to recoup their losses but the mass dumping will likely send the price going back the opposite way.