There's only one way I can see Final Fantasy XV being on the Switch:

Whenever Nintendo introduces the Switch's equivalent to the PS4 Pro or Xbox One X, with hardware, architecture, and graphics that's close to or at the same level as the base Xbox One and PS4. They're going to want a game, a very big game that gamers care about to demonstrate what that refined, upgraded Switch model is capable. I think Final Fantasy XV would be the perfect candidate.

Nintendo would go after Red Dead Redemption 2 over FF XV if they were trying to appeal to the masses.
Like Skyrim on Switch.

If we see nothing at this year's PSX I have to disagree. That means they aren't even at the point of showing massive progress on PS. But in terms of overall possibility yes, it does have a chance. Issues will be with texture res and frame rate but software engineering wise it is possible as well. They could at least put the original version on the Switch, I'd be fine with that actually.

Ff15 only works on the upgraded switch? But all ps4 pro and Xbox one x games can be played on the base ps4 and xbox one as well. 

That's because all games made for the PS4 and Xbox One are developed for the base systems and then are enhanced on the Pro and X models to take advantage of their extra horsepower. 
But, the current Switch isn't as powerful as the base PS4 and Xbox One. It's barely above the 360, PS3, and Wii U.  There's no way the current Switch model would be able to run a game like Final Fantasy XV. 

It's possible, but I have my doubts. The game looks quite a bit too demanding for the Switch to handle. I dunno if downgrading would be an option either.