So at end of April 2017 we had 58M sold PS4 right?
That would and with the current baseline it should probably be at wee bit over 59M. Possibly another half a million but likely not more than that.
So 42 million short of the Wii. In comes the Switch, in comes the Scorpio and they take market share from the PS4. I think that's a given, like it or not. PS4 has pretty much had a bump free ride so far... it wont be as smooth from now on.
I think it will happen in 2020. Not earlier than that. Possibly leg it out over 2021 but I do think it will pass it at some point. But people saying 2018.. I doubt it.