With infinite memory and "no cost memory allocation" you'll be likely correct, but in real world production loads you'll hit those limits quickly and then the immutability is likely getting in the way. Imagine just keeping a large (possibly compressed) bitmap (TBs in memory) to keep track of subscribers in your queue which is what many distributed queues use - how would you do it in immutable fashion? Copy the whole bitmap on each small change? There are simply many problems where immutability is the wrong answer if you want anything resembling performance.
Absolutely, that would be one of the cases where "most of the time" is not "this time." Although for most developers/applications, I would imagine keeping TBs of bitmap in memory isn't a common task. Fetching some rows from the DB and transforming them into some sort of HTML or JSON probably is
