The problem is sometimes you lose a lot of functionality when you allow for that flexibility. For example, let's say you want your functional language to allow

The problem is sometimes you lose a lot of functionality when you allow for that flexibility. For example, let's say you want your functional language to allow for side effects. Immediately you've lost locality: the explicit arguments of a function no longer guarantee their results. Or maybe you want to allow mutability: there goes thread safety. The fewer hard restraints a language can impose, the less lifting the language can do on your behalf. Give up on Lisp's S-expressions, and the macro system becomes vastly more complicated. Likewise, a language has a limited amount of space for syntax before you end up a mess. So languages adopt a paradigm, and optimize the language for that paradigm. Functional-style Java is bloated to hell, because Java is built for Object Oriented programming. Frankly, I'm of the opposite mind of the quote: the middle ground is often worse than either extreme. I'll happily write Object Oriented C#, or Functional Haskell, over a tepid mess of C++.