I suppose it’s tricky to apply an FP approach when you get closer to the external interface, it’s moe suitable for the more internal stuff. But when you combine it with reactive approaches into FRP you can do more, and there are more complex examples to be found.

But I’d say in general that FP is mostly just an alternative to OOP, maybe even leas than that, not a complete paradigm for writing complex programs. At least if you use a mainstream language, then FP is probably something you do pretty locally.