> But is it not easy to opt out of in C, C++, Zig and Rust, by simply not using the types that use reference counting?

In C, Zig, and C++ sure. In Rust? Not without resorting to unsafe or to architectural changes.

> And how does your performance analysis consider techniques like arenas and allocating at startup only?

Allocating at startup only in itself doesn't say much because you may be allocating internally - or not. Arenas indeed make a big difference and share some performance behaviours with moving-tracing collectors, but they can practically only be used "as god intended" in Zig.