But then why did you include mobile in your post?
Because of Fortnite. Right now, the only difference between X1 and PS4 is the ability to play that game with Switch users. Both support mobile and PC crossplay. I was simply pointing out that the Switch is literally the only factor in this debate.
However crossplay in the next get will not have Switch support outside of indies with less demanding visuals as it will not be able to hang with the two newer consoles on a AAA setting. Fortnite is no longer a factor because those still playing are not going to buy a new console to play the same game.
It is also important to remember that mobile tech is one of the fastest growing in the industry. By the time the new consoles drop newer phones will be able to handle higher teir indies that the Switch might not be able to. However in this case, a heavily downgraded version is still a factor.
The only way I see Switch remaining relevant in crossplay is if another indie blows up like Fortnite did, which only time will tell. It would also have to happen right around the launch of the PS5 and X(2) while sales are on an equal footing. The thing to remember when tempering expectations however is that Fortnite's success is the exception, not the rule.