What the factorial example proves is that the set of recursive functions that can run in constant space is larger than just tail-recursive functions. You're ri

What the factorial example proves is that the set of recursive functions that can run in constant space is larger than just tail-recursive functions. You're right to say that TCO guarantees increase the expressiveness of a language. If we could provide the same guarantees for an even larger class of recursive functions, it stands to reason our expressiveness would increase even further!