It doesn't have Kernel#eval. It doesn't have Kernel#send. It doesn't have Kernel#binding. It doesn't has Proc#binding. It doesn't have Kernel#instance_variable_get/set. It doesn't have Binding#local_variable_get/set. It doesn't have BasicObject#method_missing. It doesn't have BasicObject#instance_eval. I could go on. All these methods have extreme far reaching non-local implications on the semantic model and practical performance, and specifically defeat many conventional optimisations.

> To the point where many Ruby scripts are completely valid Crystal, or at the very least require only a few changes.

You can't even load most of the Ruby standard library without these methods!

And it doesn't matter if you use them or not. They're still there and they impact semantics and performance because the fact that you can use them affects performance. You can't even speculate against most of them as they're so non-local.

Rails and the rest of the mainstream Ruby ecosystem fundamentally depend on them.

> and are also semantically very close

Sorry I super disagree with this. They look similar. Dig into it just below the surface? Start to model it formally? Not at all. Method dispatch, which is everything in Ruby, isn't even close.

(Again, Crystal's great as its own thing, it's just not similar to Ruby's semantics. If you don't need Ruby's semantics or you can replicate them at compile time then maybe it's perfect for you.)