This is true, even Haskell supports imperative programming and mutation (very well in fact) but languages do need defaults. Given that even OOP thought-leaders

This is true, even Haskell supports imperative programming and mutation (very well in fact) but languages do need defaults. Given that even OOP thought-leaders argue "favour immutability", I would argue that FP languages offer a better set of defaults out-of-the-box (e.g. persistent collections, libraries for working with immutable data, concise syntax). Mutation is extremely useful and powerful, but if it's the default idiom used absolutely everywhere, then program composition and reuse will be harder to achieve.