As a relatively novice programmer who's worked in tech for decades but not as a software developer: I take issue with the idea that you need to write considerable amounts of Rust and C++ for these statistics to be expected. In fact, despite Rust's initial vertical learning curve I'd say that any junior developer trying to implement anything with any degree of complexity at all in Rust and C++ would see the benefits.
At the very least, the fact that IDE integration can tell you all kinds of stuff about what you're doing/doing wrong and why accelerates things greatly when you're starting out.
The problem with junior developers is that Rust will be incredibly frustrating to learn by perturbation, because the compiler will reject most random changes to the code. Which is the point of course, but C++ will compile programs which then crash, giving you a very misguided feeling that you’re making progress, but this is very important in the process of gaining new skills.
I don’t see a way around it, programming without garbage collection is hard, Rust makes it very clear very quickly, which is also the point, but this is at odds with making the learning curve accessible.
> The problem with junior developers is that Rust will be incredibly frustrating to learn by perturbation
Yes, this is the biggest issue with Rust that I've seen; most language will let you do something wrong and then as you learn you get better. Rust will refuse to compile if you're not doing things correctly (and normally I would put 'correctly' in quotes but correctness in Rust is well defined).
The first time I tried to experiment with learning Rust was a disaster. I just wanted to decode some JSON and filter it, but -- oops! -- I don't own that variable. Okay, well I can pass it somewhere else mutably, right? But then that function does the work and returns something that... what's a lifetime? What's a 'a mean? How do I... screw it, I'll go back to Python.
Eventually, after the tooling and the tutorials got better I came back to it and really enjoyed what I've seen so far and even rewrote one of my own personal tools in Rust[1] to experiment with. It's nothing impressive, but it was fun to do.
The logic in my comment wasn't that you need to have written considerably amounts of code to be expecting this, just that to not be expecting this would make me think you hadn't. If that makes sense.
On your second point, I think IDE integration for C++ is similar as it is for Rust. Just Rust errors and tooling are a million times better regardless of IDE.
Oh, the more junior the developers, the quicker they will get any benefit. That's common for any language that enforces correctness, but the C++ vs. Rust comparison isn't even fair; C++ is an incredibly hard language to use.
At the very least, the fact that IDE integration can tell you all kinds of stuff about what you're doing/doing wrong and why accelerates things greatly when you're starting out.