A relative pointer with limited range is something that should rarely exist, and all too easily it can end up with a base that is no longer guaranteed to be adjacent. There's a lot of bonus landmines compared to just having a 64k element limit.
By "landmine" I mean situations where it gets very unpleasant to use because the data isn't guaranteed to be close enough to fit into one of those pointers. Even if it's true when the code is first written, minor updates might change that. I don't think something like a borrow checker can do much to help with that.
Let me put it this way: 16 bit indices are already rarely worth it. 16 bit pointers are 10x the trouble for 20% more situational coverage. It's not worth it. They are not similar levels of useful/terrible.