That works both ways. No need to get an Xbox either.
I'm one of the only PS4 players in my friends/family group. Maybe I would have stayed on the PS4 version and spent my money there if cross pay was a thing. Instead, I started playing on the Xbox version of Fortnite and spent money on micro transactions there.
At worst, Sony don't lose OR gain anything by allowing cross play. But at least their third party partners gain. Maybe they could think of them?
Your situation is the minority.
It is probably more likely it's the other way round.