I know Rust has these features, but they are intended to be features of last resort and it shows. It's not a criticism; Rust pushes people relentlessly towards safety and performance, and is extremely successful at that.

I am imagining a language more like Pony, but with less of a focus on the actor model. I could use a language like that to write a high performance game engine with no GC pausing issues, and then write very high level gameplay scripts on top of it in the same language.

You could do that in Rust, but the Rust game engine space has already made it clear that most people feel the need for a scripting language.

Box

Now try to do that with a trait that isn't object-safe...

I get your point, these things make fighting with the borrow-checker a little bit less annoying, but Rust is complex. I'll happily accept that because I value high code-quality (to that point that I rather invest more time to get things right) but when that is not the goal and you want something higher-level and strongly-typed there are alternatives that work better (I'm just talking about the language itself, ecosystem alone for me is yet another pro-Rust thing)