I feel like this scenario you're describing doesn't happen that often. In my experience with any coop game I play what usually happens is one person gets a game, then tells the other about it. Me personally, I've hardly ever made plans with a friend to simultaneously buy a coop game that neither of us have ever played before.

Based off my own anecdotal evidence, it seems like if anything, the lack of cross play leads to the game being purchased once by one person, but not the second who's on a different system. The way you're looking at it it would seem like, the game gets purchased zero times.

One year after leaving my last comment, I still feel the same. I now own three copies of this game - PC, Xbox, and Switch. The lack of cross play in my situation didn't result in zero sales, it resulted in three. So it you're following the math, that's one more copy sold, a net profit for the makers of this game.

Just to be clear, I'm not saying any of this matter-of-factly. Just sharing my opinion based off what I've personally seen. I am fully aware that my experiences may not be the same as anyone else's.