Allocating within the given arena is simply an option. You can still be a component owning an arena, but delegate the allocation to an ancestor/sibling context/arena if you want that allocation to outlive you.
But like all the other things listed it's not perfect.
For example if I just pass a unconstrained access to a other components arena to a component you can no longer "guarantee" that the component doesn't leak anything as it could use the arena to allocate things besides the thing for which it was passed it to the component. So now you need to have additional steps to assure this isn't happening if you want to make sure as much as possible that everything is compliant. Through depending on what you do this aren't necessary complicated additional steps through it really depends.