(Of course making it performant, not rendering/“simulating” everything and the like is exceedingly hard, but it is true that it is more of a “closed world” as opposed to some other areas of software development.)

Although arena memory is casually described as "allocate one huge chunk of memory up front," you are not literally only allocating one block ever and praying it never runs out. If you run out, you allocate another block. The point is that you don't call malloc for every string, object, list, etc. Adhering to this largely eliminates the need for RAII. What about this doesn't scale?

Personal anecdote: I'm building an IDE, where literally all of my data is provided by the user, and arenas have worked perfectly. I don't think I have a single destructor except for dealing with things like file descriptors, etc.