No reason to turn down the extra sales, there's no guarantee everyone who bought it for XB1 would have bought it if it were only on PS4
I think these things are incredibly hard to determine. Some of the things one needs to take into consideration:
1. How many people are ps4/xb1 owners and would have bought it on ps4 if that was the only option (i.e. your point)?
2. How many people would have bought a ps4 for it?
3. How would it being an exclusive have affected its sales because of the hardcore fans or the fanboys?
(one needs to consider the brand. Weren't there reports of Japanese players considering XIII being available on xb360 a betrayal and refused to buy the game? Don't know their numbers but the company sure doesn't want anything ruining their image, even in the eyes of idiots if there are too many)
4. What kind of push would have Sony provided that would have saved SE money for ads?
(Sony could have made it the holiday bundle and sales would have been much higher)
5. How much did it cost to port it?
6. Would it have come out sooner if it weren't for the xb1 port and if so, would it being available on Black Friday have affected its sales?
edit: 7. Did MS pay SE to port the game on xb1 and if so, how much?
I'm sure there are more that I forget. Bottom line is, we'll never know and things are not as simple as presented here.