There really is only so much you can do - especially once you've got refcount churn on objects being used across multiple threads. Swift's refcounting can get truly annoying at times.

Modern, state-of-the-art, generational GC? Not being snarky, that’s literally the comparison.

There was never any way to implement a modern GC while keeping compatibility with Objective-C, which was the primary design goal of Swift.

Apple had a bad experience with GC in ObjC with libauto and learned their lesson that C and GC just don’t mix. You can never have a truly modern GC in C, because C cannot provide any of the guarantees that a GC needs to move objects around.

Using a modern GC entails sealing the language off in a bubble. Since Swift is mostly used to interface with system frameworks, you would end up paying a cost when interfacing with the system frameworks written in ObjC, making your modern GC useless most of the time.