I agree - I think MIR is trying to produce better machine code more quickly, but that isn't the problem Ruby faces! The problem Ruby faces is having to inline through ten levels of metaprogramming in order to get any meaningfully-sized compilation unit that you can optimise, and only then it makes sense to worry about code generation.
However, the reason that this is not so simple is that Ruby is a vastly more complicated language than JavaScript (I've worked on implementing both.) Ruby has an enormous standard library and most Ruby programs are just endless calls to the that library, so your compiler must be able to understand the library semantically, either by rewriting it in Ruby (not likely at scale in MRI) or by adding tens of thousands of individually optimised intrinsics (again not likely).
Ruby is digging itself further into a local optima with these approaches optimisations, rather than looking further around for a better global optima.
However, the reason that this is not so simple is that Ruby is a vastly more complicated language than JavaScript (I've worked on implementing both.) Ruby has an enormous standard library and most Ruby programs are just endless calls to the that library, so your compiler must be able to understand the library semantically, either by rewriting it in Ruby (not likely at scale in MRI) or by adding tens of thousands of individually optimised intrinsics (again not likely).
Ruby is digging itself further into a local optima with these approaches optimisations, rather than looking further around for a better global optima.