"We don't need to read and understand stuff. We just grab the code by the horns and spur it with changes until the compiler stops thrashing. Yeehaw!"
There are myriad things that distinguish long-term maintenance from greenfield development. Like transferring application ownership from one team to another, reverse engineering, adapting to the changes in external systems you integrate with, investigating bug reports and performance issues, doing monitoring, etc. etc. If your only concern in "maintenance" is making some changes while avoiding the kinds of bugs that can be prevented by a static type checker, then something seriously does not add up.
Again, the wast majority of claims made about FP on Hacker News right now (even in this thread) were made about Java in early 00s. Almost word-to-word, except some terminology. Unfortunately, back then I didn't have enough experience to spot the issues with those claims and people who did have real experience were mostly silent.
At the end of the day, sure, we all have opinions on tools and how they make our lives better. You and I may not agree that FP is the right tool, but I’m not going to make sweeping generalizations that you know nothing about large scale development and and maintenance solely based on your language paradigm choice and a few focused HN comments.