The problem is that some of those features - function composition, stream/LINQ type APIs, list comprehensions, etc. - become very difficult to reason about/debu

The problem is that some of those features - function composition, stream/LINQ type APIs, list comprehensions, etc. - become very difficult to reason about/debug when used with side-effecting code. Yes you get the conciseness but you can create some truly obfuscated behavior when used with non-functional code. Don't get me wrong - I'm delighted to see such features available in more languages. But they don't all work nicely with the typical abstractions (e.g. objects holding state) available in non-functional languages.