Depends on what you want to do. Either way, exposing yourself to more programming languages ideally with different styles and background should make it easier to familiarise yourself with others.

So, either choice is not wrong.

C and C++ are clearly different programming languages, so you are actually looking at three languages.

> Rust could also turn into something big, by the looks of it, in the next 10 years.

I wouldn't bet on that. 20 years ago, I heard Haskell and or Java is going to be huge. They are, C and C++ still are. Learn Rust by all means, probably you will be more quickly productive, and it gives you a wide range of choice of targets. That should be sufficient reasons to go that way.

> To me, it seems Rust is so much better than C/C++ in many ways: lots of Python-like abstractions, ...

Yes, but that is also an argument to go for either C or C++. You'll have to learn more. The active code-base in C and C++ is huge, and I don't see everything being rewritten in Go and Rust. Maybe you'll be less productive in churning out code, but it'll give you expertise to look and modify the existing code-base when necessary.

Not that you cannot do that when learning Rust.

In summary, I would go, with Rust, if you want to be quickly able to do your own stuff, C or C++ if you want to be able to work through existing code build "with a hodge podge of welded on features". The deciding factor between C and C++ again should be then, in what language is the code written you'll be looking at. I would say, (and others might disagree) kernel-code (OS kernel, that is), Microcontroller, low level stuff more likely C. C++ you'll find more likely in application code, numeric libraries, etc..