I think crossplay can work with PlayStation and Xbox but definitely not with the Switch. The FPS and the downgrades needed to get said fighting game on Switch would screw up the online. Would be stuttery and laggy as hell. Look at SFV on PC and PS4 and imagine that 5 times worst.
Any serious fighting game publisher would probably not touch the framerate on a Switch port.
Dragonball FighterZ is 60fps on Switch. Not only that, but it's even 1080p.
I think Namco would rather not port Tekken 7 to Switch at all if they couldn't keep it at 60fps.
They did however halve the framerate for DB Xenoverse 2, but that's not really a traditional fighting game experience.