When Sony had the market by the balls, not having crossplay with it's main competitor (and pretty much only at the time, poor Wii U) was beneficial for Sony. But now that Switch is quickly getting a very serious contender and visibly teaming up with Microsoft in Minecraft, it's going to make more harm than good.
Minecraft crossplay on Xbox and Switch?
That'd be spicy.
Switch and xbox are really showing no ability to affect ps4 sales in a significant way. And with the insane line up of ps4 exclusives and 3rd party deals on the way this ain't gonna change. Wishful thinking to believe otherwise.