If you don’t have a perf requirement like “all these things need to be in contiguous memory” then you probably don’t need a generational index anyway… it is effectively a weak reference, after all. ECS stores are optimised for repeatedly iterating over all the things, and games might have complex notions of “reachability”, but most things aren’t like that. There does seem to be a lot of “I don’t like using Rc RefCell” in object arena design that isn’t always justifiable, though nested generics don’t make for the most readable code in the world.