I spend an inordinate amount of time at my C# day job adding documentation comments about exclusive access and lifetimes and ownership… things which are clearly important but which dotnet provides little or no useful support for, even though it has a perfectly good garbage collector. The dotnet devs were well aware that garbage collection has its limits, especially when interacting with resources managed outside of the runtime, and so they added language features like IDisposable and finalisers and GCHandle and SafeHandle and so on to fix some of the things GC won’t be doing for you.

I’d happily use a garbage collected language with borrow checking.

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.

You can always use something like generational indices. They pop up a lot in ECS systems. A suitable container with an opaque index type prevents creation of invalid references, lets you check reference validity at runtime, and generational indices prevent reuse. The compiler can’t help with lifetime tracking, but that’s a problem with any shared reference type pointing to a resource with a lifetime that can only be known at runtime, eg. Arc.