You can do that but you end up with something other than what the author wrote. The use cases are different. Arenas, at least in principle, should generally be

You can do that but you end up with something other than what the author wrote. The use cases are different. Arenas, at least in principle, should generally be able to allocate anything, not just a single type. If you have a single type you would do the equivalent thing in any language, not use arenas.