That's a good point. People don't understand that functional purity in Haskell, while a nice goal, isn't a law, you can break it if you have a good reason to. IO is sort of the canonical example, it isn't there to prevent you from doing IO, it's there to act as a giant warning sign that either this function, or one of the functions it uses isn't pure.
Likewise mutable data structures while not the default, can be implemented in Haskell, they just have some caveats attached and shouldn't be used lightly. In other words, all things being equal, use the immutable data structure, but if you have a good reason to, you can use a mutable data structure instead, just be sure you know what you're getting into.
The problem is that "all things being equal" never happens because immutable data structures have worse performance if not used in a way that benefits from persistence. IMO, that makes them a poor default.
I'd rather say that 'performance+ convenience/safety-' is a poor default - minute performance improvements matter only in the bottlenecks of your code, so it makes sense to use the varsatile though slightly slower implementations as the default, and have the 100%-speed option require some explicit code as they are needed only in a minority/exception segments of your program.
:) You could say that "all things being equal" never happens because mutable data structures have worse persistence if they're not used in a way that their performance matters. But also, different defaults make sense for different problem domains.
Pretty much everything except performance benefits from immutability and persistence, which is why using anything else as the default seems insane to me.
Likewise mutable data structures while not the default, can be implemented in Haskell, they just have some caveats attached and shouldn't be used lightly. In other words, all things being equal, use the immutable data structure, but if you have a good reason to, you can use a mutable data structure instead, just be sure you know what you're getting into.