It sounds to me as though what you're saying is that when you write Rust programs which don't scale very well they don't scale very well, whereas when you write C++ programs you don't do that, I suggest learning not to do it in Rust either.
Easier to be said than done since that's one of the core tradeoffs of the Rust language. Language is forcing these semantics upon you and while it is possible to get around it with unsafe blocks it turns out to be much more difficult in the practice. So, by default Rust code will almost certainly in majority of cases going to be designed around shared ownership.
If you actually have shared ownership but in C++ you're getting away with pretending you don't, chances are that'll bite you really hard. Maybe it's already biting and you didn't notice, so Rust actually do you a massive favour.
If there is no shared ownership then inventing it so as to make your Rust slower is just a problem with you, not with Rust.
No. For 98% of the multi-core sensitive code I don't have nor I need shared ownership. While C++ doesn't force you into such semantics but provides you with the ability to do so, the Rust semantics and compiler pesimisstically do. I am going to stop here since I'm repeating myself and you're blatantly going over my points.
