I'm not sure that it's a complaint from them, so much as an explanation. It's important to realize that developers are human beings with human needs, wants, and feelings. The popularity of Lemmy is not their "fault" and the language choice is rather fundamental to the project itself. Would it be nice if some features were taken up more quickly or implemented in other ways? Yes. But others needs, wants, and feelings are not more important than those of the devs. They need to eat, sleep, provide shelter for themselves, and, importantly, do things that are not coding (for physical, social, and mental health).

The number of people who understand Rust, can code in it, know of Lemmy and want to contribute is very few. There would be More developers contributing to Lemmy if it weren't written in Rust.

And there would be more developers if more people wanted to learn Rust. The low number is just a fact to accept. If one can't accept it, there are plenty of other platforms.

Would you be criticizing them equally if, instead of Rust, they created the project using FORTRAN and made a point of mentioning explicitly that using FORTRAN was the main intent? It's just a weird criticism to me - Lemmy is fundamentally a project started so that the devs could work with Rust. You are criticizing them for their project not fundamentally being a different project. Maybe another comparison would be criticizing specialty water-based paint manufacturing for using a water rather than a VOC-solvent for water-based paint - they're not trying to make other types so, the criticism doesn't make logical sense.