Code being in a simple state also matters with the efficiency of agents ime.

It is not just ergonomics but the point of using rust is to have solid abstraction and a lot of high quality libraries to use.

If you are doing static allocation + arenas then you can't use nice abstraction, you can't use libraries, you have to write a ton of unsafe code because of these reasons. Then using rust is actually working against you so there is no point in using it.