Is this view of FP based on actual experience maintaining a non-trivial program written in an FP language? In my experience, FP doesn’t necessarily add a lot of unnecessary complexity. Sure, languages like Haskell are perhaps initially a bit more abstract when learning them, but once you know the basics, you can write pretty straightforward code in it. You can also do crazier things, but there is no need for that in most software.

Keeping a functional style, regardless of the language (although FP languages lend themselves better to this) can help in keeping code more decoupled, since you have to be explicit about side effects.

I think that both FP and imperative languages have places where they shine, and I freely switch between them depending on the project. Given how much some imperative languages have recently borrowed from FP languages, I think that this shows that functional programming has some significant merits.