I am really surprised sony is considering it.
I don't think they have much of a choice. Having the highest selling console out of the compatible consoles doesn't provide any real leverage when the combined userbase of all compatible platforms is much bigger.
Games like Fortnite are harbingers of a changing gaming landscape. Instead of a console being the platform, games themselves will be the platform. Previously people had to pick their console based on what their friends owned, so this was beneficial for console manufacturers because the closed environment created a snowball effect. But crossplay flips this premise on its head, so a closed environment (i.e. not allowing crossplay) becomes a disadvantage for console sales as the PS4 in this specific example is the worst platform to play Fortnite on.