Some languages don't really intrude on one another's territory — e.g. not that many people are rewriting Ruby programs in Rust — but some very directly compete.
So if you want to convince someone to use F#, you have to convince them it's significantly better than some other closely-related language. And that's hard!
I have a strong suspicion that the next decade will see a reduction in programming language diversity. JavaScript/TypeScript and Python will become even more popular, to the detriment of everything but Go, C++, and Rust.
Platform-specific languages like Swift will persevere, as will Java, which is unkillable, but the vast array of languages will become less vast.