Then you need to retain the personnel who give you that capability. Because they are rare, in a field in which 99%+ of developers only glue together NPM or PyPI packages. (And many use Web search or, now, Claude Code to do the glue part.)
If I founded a startup doing mostly Web-like server backend work, I'd consider doing it in Racket or another Scheme, and then using that as a carrot to be able to hire some of the most capable programmers. (And not having to bother with resume spamming noise from hardly any of the 99%+ developers, who will be pounding the most popular resume tech stack keywords instead, because their primary/sole goal is employability.)
They're niche because they're doing weird, interesting things. Like creating their own VMs to support funky features. So nobody wants to depend on them: low bus-factor.
They can do weird, interesting things because they don't have a large user-base that will yell at them about how they're breaking prod.