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.
