Its shitty but the ball is in their court.
Sony is thinking short term - "If we open up on Fortnite, we'll sell less content now through our digital store". So no one at Sony is willing to fight for this. It won't make any charts go up for the next quarterly earnings.
But the long-term view is that they are teaching an entire new generation of gamers that "Playstation is the worst place to play Fortnite - the world's most popular game". That is going to be a disaster the next time new consoles are released - just like how Microsoft made similarly dumb moves when they first launched the XBox One.
If we assume PS4 is not the dominant platform then your assertions might be true, however if PS4 is the dominant platform then they're absurd.
Sony is teaching the same lesson as Nintendo, Microsoft, Sony, Atari. It's best to buy the same console as your friends.
Of course that's not to say that any mobile user who dips into a free-to-play game has the same long term customer value as a hardcore console player so I'm not sure how that math works out. But it's worth keeping in mind that the player base of any console is dwarfed by phone users and that's a funnel of millions of new gamers who see the PS4 as the only platform that "sucks for Fortnite". I think that could have long term market effects.
Rephrased: The good thing about dinosaurs is that they go extinct. Imagine 1500s humans living with dinosaurs. Humans would be extinct.
So let Ms, Sony and any other dinosaur keep shooting themselves in the foot.
In the long term, neither console maker wants cross-platform support. They want to create a situation where everyone has to have both the Sony device and the Microsoft device. That's an equilibrium that both companies are happy with. Pure cross-platform play ends in consumers not needing both devices and hence total sales will drop.
When the "pendulum swings" and Xbox has a dominant generation, expect Microsoft to reject cross-platform play.
Companies change all the time, though. The next head of XBoX could go back to the Mattrick days and completely burn all the good will they have. But, as long as Phil and Nadella are around, I don't think they'll be blocking cross-play anytime soon, even if they do become the dominant platform.
Microsoft had 360-PC cross-play in a very small number of games (under the now much maligned Games for Windows Live banner and DRM regime), such as Shadowrun (FPS). Microsoft wanted some consistency in the network stack, and GfWL was a part of that attempt.
Apparently Trion when building Defiance (MMO related to TV show) asked both Sony and Microsoft for cross-play (and technically demoed it), but was rebuked by Microsoft's "network experience" concerns, which at the time were at least somewhat valid, particularly compared to what Microsoft was trying to do with Xbox Live and GfWL.
A couple titles (one of the FF MMOs, 14 was it?) just built in cross-play between 360/PS3, didn't ask, didn't really advertise it, and mostly were successful and not shut down.
On the one hand, Microsoft downplayed cross-play with Sony in the 360 generation for fear of "filthy non-paying gamers with no proper user accounts" ruining the Xbox Live experience, but on the other hand Microsoft was very excited for cross-play as an opportunity on Windows.
Microsoft's change in stance here on cross-play over the two generations could be entirely attributed to lessons learned from GfWL's failures, and doesn't necessarily have anything to do with dominant position on console one way or the other. A lot of Microsoft's current excitement about cross-play doesn't look that different than from the 360/GfWL era, beyond, of course, branding changes (Microsoft Store, Xbox Live (on Windows)) and some relaxation/evolution on network accounts and DRM/anti-cheating (for instance, Xbox Live on Windows is "free" now, Gold status is no longer required for PC multiplayer; Microsoft Store is little bit of a friendlier DRM regime than GfWL at its worst; etc).
https://www.gamespot.com/articles/final-fantasy-14-director-...
"One of the issues Yoshida noted, however, was that many companies don't consider how updates to the way online services are operated and regulated can impact an MMORPG.
"Those can become a hurdle when we consider operating FFXIV for an extended period of time," he said. "So when I talk to those first-party companies, I ask them, 'Do you have the capability to prepare for that, do you have the resolve that you're going to make sure to take responsibility and take care of those, do you have that willingness?'
"If we are able to come to some sort of agreement, a handshake so to speak, or if it does end up being that unfortunately we can't do a handshake with Final Fantasy XIV, either way we'll make sure to communicate with our players. But we have been tenacious--we've been trying to keep at it and be persistent about our conversations.""
Considering that cross-play between Microsoft, Nintendo, and PC is supported for multiple games now (Rocket League, Minecraft, and Fortnite), I think it's safe to say that Sony's the one holding back cross-play as a whole. As in to say, even if the above issues were fixed, I wouldn't be surprised if Sony said no anyway.
But I totally agree that Microsoft would be doing exactly the same stuff if they happened to be the market leader.
To be honest that is why a few friends and myself got PS4s. We had quite a few friends on the platform and I already had an xbox, but I wanted to play with them so I got a PS4.
What's interesting is that you can cross play with PC and Mobile on PS4 though. It sounds like they just don't want to work with Microsoft or Nintendo.
If you look at the global sales, then yes, the PS4 is stomping the Xbox One into oblivion at the moment. If you look at the US sales, the difference between them is much smaller [1]. If Sony enables crossplay, they will probably lose a lot of sales to Microsoft in the US because they lack the overwhelming popularity that they have in Japan and other markets.
This article [2] shows just how poorly the Xbox sells in Japan - the week the Xbox One X launched, only 1,639 were sold. Only 49 units of the less powerful Xbox One S were sold. For comparison, almost 30,000 PS4 and PS4 Pros were sold. Even the PS Vita beat out the Xbox, with around 3,000 sold.
[1] http://www.vgchartz.com/article/271063/ps4-vs-xbox-one-vs-sw...
[2] https://kotaku.com/here-are-the-xbox-one-xs-japanese-sellout...
Not that I appreciate their way too (supposedly the anti-piracy / console-locking measures in Switch are first grade?) but this would put pressure on Sony to possibly stop blocking cross-play.