Personally, being a Rust enthusiast, I think that Rust's benefits don't just unleash when you think about performance, but also when you think about large codebases. Such codebases are usually the scene for refactorings. In a dynamic language like js, these are extremely hard to pull off. In Rust, the compiler won't stop yelling at you until you have cleanly finished the refactor. I love refactoring Rust codebases, even ones I didn't write, while in C++ or other languages it's extremely annoying.
And note that while C++ enthusiasts might have believed that everything should be C++, not everyone followed them. Many people still stayed with C. Many still do even with Rust around. But there is a small group of people for whom Rust is such a great improvement that they take the portability cost and switch to Rust. Linus is heavily anti C++ for example, but he's open to adding Rust to the kernel.
Neither Facebook, nor Instagram or Slack are primarily built on C++ or Rust though. And many other products that I can't as confidently remember from the top of my head aren't.
Only later did they introduce Hack and HHVM, which compiles a PHP dialect in a just in time fashion. My point is that once you reach scale, and your computer bill becomes significant enough, you'll really want to use efficient languages.
Now it's Rust based on what? Uber backend is almost entirely built in Go. And so are many other services handling billion plus requests a day written in other languages.
Perhaps Rust advocates know something that Uber engineering does not.
And for the record, when Uber got started Go wasn't available either.
What's a better general-purpose language for this mythical "average" developer? Java/C#? Rust has the same level of memory safety, and adds data-race safety that these languages do not have out of the box. Go is in the exact same boat, btw - memory safe for sequential code, but concurrent code is tricky since unsafe shared access is ubiquitous in real-world Go code. Haskell, Agda and Idris could provide more safety but are not approachable to the "average" dev.
You (rightly, I think) place Idris (and Agda, although that's more a proof assistant than a PL) beyond the reach of the average developer, and also, apparently, Haskell -- arguably a simpler language than Rust, but never mind -- but not Rust. And that's my point and, I guess, our point of disagreement. If you think that, on that spectrum between, say, Python and Idris, Rust is at a good level for the average application developer to the point where maintaining high-level applications written in Rust would ever be cost effective, then I think you are out of touch with the realities of the software industry and the economic forces that shape it. I thought the exact same thing as you twenty five years ago: C++ was obviously the right language for the average developer -- even more so than, I don't know, Delphi or Visual Basic -- wasn't it?
> What's a better general-purpose language for this mythical "average" developer?
I don't believe any low-level language will be cost-effective for application development for the foreseeable future, if ever, and the average developer is anything but mythical (I mean, embodied as a single person, perhaps, but I'm talking about teams).
> Rust has the same level of memory safety
It doesn't (due to much more prevalent reliance on unsafe as well as FFI), but that's very much beside the point because, as you acknowledge, safety is not what it's about.
From a PL research point of view maybe.
But from a POV of what is simpler to use for a average or new programmer, Haskell is far worse as far as I can tell.
> due to much more prevalent reliance on unsafe as well as FFI
Where? In embedded programming? Or programming from primitives (which I would argue a average programmer never should do, there are existing libraries)?
In some use-cases you always end up with a FFI, but that is also true for other languages.
Besides that usage of unsafe is both strongly discouraged and uncommon.
There are a lot of rust use cases where using unsafe code is never necessary to a point where people simple ban any direct usage of unsafe code in their project.