Back up what you're saying. Rails built a lot of extremely successful companies. It is not hard to scale or maintain if you are competent. If you scale beyond the capacity of Rails, you are Twitter and it's ok to move on.
I am seeing good amount of comments saying "Twitter did it", this is simply a logical error (survival bias). Moreover, I think Twitter would of been successfully even without using RoR.
I think your statement is patently false.