I don't really focus on Web development (though I've done a little in various languages when it became necessary to solve specific problems), but over the years, for software development, I've become pretty conservative about adopting new programming languages, frameworks, libraries, and so forth when they come out. I have seen a very large number of things that were new and trendy vanish in the wind after people put a lot of effort into coming up to speed on them.

Once there's a large installed base, though, they're gonna be around for a long time to come, because software projects have committed to them.

None of this is to ding Rust, which Lemmy uses. I've never written a line of Rust myself, don't know the ecosystem. But I don't think that PHP is bad just because it's pretty mature.

Also, related side note: Reddit was originally implemented in some Lisp variant, probably in part because Paul Graham, who was involved in some early funding, is a huge Lisp fan. There isn't that much web dev happening in Lisp, and Team Reddit later had to go back and reimplement it in the more-widely-used Python, on the Pylons framework. That doubtless cost them a lot of dev time.