[RyanCavanaugh]

Maybe this is simple in implementation, but it's definitely not simple in developer experience.

You grab some code in one of your old projects for implementing a binary search. Can you copy-paste it into a new project that targets a newer language version?

The question isn't as simple as "does it have syntax errors", because we're talking about changing semantics here. Given a set of semantic changes and a piece of code, figuring out (either as a human or a computer) whether the observable characteristics of that code have changed is somewhere between vexing and impossible. It's entirely possible, for example, that your code encounters changed semantics, but not in a way that changes the actual behavior of the code.

In this world it just becomes very, very difficult to reason about extremely common operations; it'd be a constant source of frustration. There's a good reason you rarely see languages versioning their behavior in impactful ways.