There isn't (outside your friends not being able to play with you but I'm sure there are more PS4 Fortnite players than Xbox and switch combined.) and from the July and August NPD (for the US) and Sony's financials their consoles sales weren't affected either.  

Wasn't crossplay available between PS4 and PC already??? Those are the main platforms alongside mobile for Fortnite. XBO and Switch are just a small fragment.

On topic: If Sony didn't want to do it before it was their choice, if they want now, it's their choice too. If they're doing it now, it's because it must be beneficial to them too. They're probably saving controversy because i think annoucements for PS5 are getting close and they don't want anybody to use it against them like the controversy with XBO launch.

I wonder how this work with other games though, because people are now  going to expect cross play in every game....

It is interesting how certain fans will react, as this site was the only one that I saw actively trying to defend Sony closed platform (on the ones I am on). Wonder if they flip flop and go Sony always listens to the fan or something else. Also this is just one of many games that should have crossplay, so hopefully this is a trend moving forward and not just band aid on an issue.  

I guess they felt the pressure, good for them that they finally came to their senses

See! When Gamers unite we can accomplish anything! Thank you Fortnite devs for LISTENING to the PLAYERS!

Or if your friends play it on a different console as you

The majority of the playerbase would be PC, mobile and Playstation themselves which all had crossplay with each other.

https://blog.us.playstation.com/2018/08/13/playstation-store-julys-top-downloads-3/

Second sentence doesn't make sense all Fortnite is still the top downloaded Free to play game on PSN even after the hoopla (and has been for years) meaning that new and old buyer alike are still downloading the game.  Also Matt Piscatella from NPD mentions that Fortnite is still the main driver of sales for consoles during this past summer.

I'm not sure how I feel about this tbh. If they added it to Final Fantasy XV comrades I'd be happy I guess.

That's what I hope. Such stuff never really affects sales, the VoiceChat debate also doesn't hurt Switch-sales. So things that are beneficial for the customer (in this case the player) but not for the company are usually omitted. But Microsoft and Epic were taking shots at Sonys reputation, that's why I think Sony reconsidered. As crossplay is beneficial for gamers I hope the public debate now makes it impossible to deny it in the future, as it is simply expected. In that case the companies have no chance. That includes also MS, if they get in the situation like they were with the 360.

This is an outrage! I won't stand for them allowing riff raff to share our servers! I'm selling my PS4 and breaking my copy of Fortnite in two. Canceling my PS Plus account as well.