> My angle is systems programming, and there, it absolutely matter. If you are performance sensitive, then you try to avoid crossing the user-space -> kernel boundary more than you have to. Eg, ask for lots of memory, manage with arenas.

This gives the misleading impression that ordinary memory allocators are materially different from arena allocators. They aren't. Both types of allocators first ask for a big block of memory from the kernel, then dole that memory out in userspace. There's no need to cross the userspace/kernel boundary more often than you need to, especially when you consider that you can replace the standard platform allocator with whatever you want.

To wit, C doesn't emphasize arena allocation anywhere near as much as Zig et al do, and yet nobody alleges that C is somehow less suitable for systems programming than these languages. Have you considered why that is? Because, for the most part, arena allocation doesn't make a significant difference, and in the places where it actually does make a difference, you can trivially build an arena allocator on top of the standard allocator.

Agreed. IME the main reason to choose arena allocators is for correctness, not speed. They make similar time/space tradeoffs to garbage collectors in that they grant higher allocation throughput in exchange for more memory usage.

The perf argument against RAII is very abstract and is less "RAII causes bad performance" and more "the kind of design that leads you to reach for RAII is the kind of design that's bad for performance." There exist similar hand-wavy arguments against many other C++/Rust features.

More generally, "you shouldn't even want that" is basically a meme at this point in programming language design. Every new-ish language has some version of it.