I think "they didn't write the songs or play any of the instruments" is often a false argument - there have been plenty of entertaining singers and groups that's also true of, and it also willfully misunderstands that pop music is often all about the overall package. As it happens, their names do appear in some of the writing credits. Not much, admittedly.
They've had some very good catchy hits - I still think Call The Shots is one of the classiest hits of recent years - but that's not solely down to the writers, musicians and producers; people might think "Oh, anyone could've sung those" but it wasn't anyone, it was that particular group, and that's what helps to make up that specific package. And it's worked pretty well so far.