>Except it's being used for serious development today
No, its being used for pet projects by people. Serious development = major companies using it in backends.
>So one of the most widely used applications programming languages in the world?
Because of CS programs, and legacy software written in java. Java has a community dedicated to pushing theoretical CS concepts into the language (much like Rust), while allowing things like a logging library to fetch code from anywhere on the internet and execute it, by default (which I would bet on would be the future of Rust given current trajectory)