Sure, Rust is not going to replace Java anytime soon (probably not at all). But for a language that has seen its first stable release only four years ago [1], Rust is a wildly successful systems programming language.
[1] Before Rust 1.0, Rust was not really usable in production due to very frequent changes to the language.
I'd say given Rust's age it's popular, and since it is the first contender for a lot of spaces where no other languages have made a serious bid in a while (embedded and real-time systems with higher safety guarantees haven't seen much love since ADA).
It will probably not supplant easy super popular languages like python. But I believe it might find it's niche. A lot of other languages don't have such a USP.
Yea, I definitely don't think it'll ever be as hugely mainstream as "easy" languages - take Go for instance, but it has huge potential and I'm a big fan of the language (I've switched entirely to it).
It may be possible that another language can come up with a more elegant solution for the memory safety features of Rust, but until that happens I think Rust will gain a lot of ground. It just won't be the language people turn to unless they have a use case explicitly for Rust (or rather, something that disqualifies Go/Python/etc to them).