this particular arena allocator does allocate objects of different types, almost arbitrary types in fact:
fn alloc_uninitialized<T>(&self) -> NonNull<T> { ... }
Conventional semantics have to hit reality eventually. Long-running programs can't just alloc forever without free, hence an arena not being the only allocator in most long-running programs.
if you're running low on memory, switching to an arena allocator is probably a bad idea, because it will usually increase your memory requirements (by delaying deallocation longer), not decrease them