Will Rust keep growing? Yes, I think so.

Will it turn into a C++-like monster? I don't know. Maybe, but when it comes to C++ it always feels like its "monster" status is largely a result of previous mistakes or early design decisions piling up, and causing trouble (eg. C-style arrays, exceptions, implicit conversions, fiddly sum types rather than first class support, no pattern matching etc.).

Rust will grow large, and probably complicated, but the real issue with C++ are all these past mistakes and a shaky foundation, and Rust is doing much, much better in that regard. Time will tell if we'll eventually look back some of Rust's design decisions as bad or unwieldy (probably).