Can someone please help me understand what someone is thinking when they say that it's "very stupid" to think that Bitcoin won't forever be the most successful token/blockchain and that a scenario where there are multiple useful tokens in existence is the worst thing that could happen.

A thinking person wouldn't say this; A religious person would. Additionally I think this article would do a great job explaining: https://www.dashforcenews.com/sorry-dash-youre-never-getting-love-bitcoiners

Most of the people who are left with BTC are the kind of people who are at least partially BTC maximalist. Most other people left the ship for better alternatives with brighter future.