EOS isn't out yet so we'll see how that works but I see your point. No matter what I try to think of to counter your argument I can't.
My original thoughts were that one of the 3rd party apps may be successful in every way Steemit could not be (and evidently does not wish to be) but that is unlikely. And that would be on a relatively minor scale, nowhere near the level of Facebook or the like. We don't have the professionalism for that.
But yeah, you're correct and pretty much predicted the future there.