"when was the last time you used a piece of software that truly achieved something useful on its own rather than solving a perceived problem that only exists because of the idiosyncratic nature of the web and cloud stacks?"
Go and Rust. Probably unpopular opinions, but I'm very glad those two languages are open source.
I'd bet that this outlook is the sort of narrow-sighted, can't-even-understand-the-question sort of thinking that the person you're responding to had in mind when asking the question—as what not to focus on when talking about the successes of FOSS. That even with the point made in a very straightforward way it gets responses like this is a huge signal of what sort of problem we're dealing with.
Go and Rust amount to infrastructure, not software that "truly achieve[s] something useful on its own".
Go and Rust. Probably unpopular opinions, but I'm very glad those two languages are open source.