[pron]

I agree that all might be too high a bar, but most is too low, because even if most of your objects fall into categories 1-3, sufficiently many objects in category 4 would still make your life miserable. Furthermore, it's not like arenas take care of too much because they still require some careful thinking. E.g. Rust's lifetimes don't automatically ensure a correct use of arenas in all cases. A language like Zig is very friendly to arenas, but you still have to be careful about UAF.

Now, I know very little about Jai, but bear in mind that its author doesn't have much experience at all with servers or with concurrent software in general, which is 1. where objects with uncertain lifetimes are common enough to be a serious problem, and 2. a huge portion of software being written these days. Games are a domain where it's unusually common for nearly all objects to have very clear, "statically analyzable" lifetimes.