Comment by pornel

There was also Cyclone before Rust, and Checked-C in the meantime. The concepts like regions and affine types existed long before Rust, and Rust started out by copying from older languages (http://venge.net/graydon/talks/intro-talk-2.pdf).

It's not enough to have just a proof-of-concept compiler that could match Rust's checks — that's where Rust was 10 years ago. Rust had time to polish its compiler, expand tooling, integrations, platform support, attract contributors, grow userbase, create learning materials, etc. To displace Rust of today you don't need to just match the old starting point, but offer something better by a margin large enough to offset the cost and risk of switching to a less mature language. That's the same problem that Rust is facing when trying to displace even more established C and C++.