I don't see that WFP will ever completely take over the job of ladders as there are some jobs I know I won't be able to do with it. Some first cleans I feel will only ever be able to be done by getting up there and getting your elbow behind the tools, especially the one like you describe. I've had a few whose uPVC windows have been in for five years with what looks like no cleaning and in my view there's no way the first clean would be successful with WFP. But then you account for that in your pricing. Some first floor flats that I do are above ones I don't do, so I'd have to trad those.
However, I'm speaking as one who is hoping to go WFP later this year and has only limited experience using a freind's equipment, so the voice of experience would be welcome on this one as I too have wondered about those annoying fly/spider spots.