If crossplay is implemented, I can guarantee 5 purchases. Friends and I split on platforms since our crazy Wipeout Fusion sessions. If Grip is cross-platform, both PC and PS4 friends will buy it. Might have one that buy it on both lol but that's irrelevant to the current discussion.
Cross-platform games are so rare you will gain high visibility amongst players. But I guess if you never started with it in mind, that's probably too late to hope for it as it requires (I guess, I'm no pro at this) all the network stuff to be handled on your servers instead of Steam and Sony's (I'm not far from truth right ?)
So, planned or not ?