To circle back to FF7 Rebirth, I really don't think it'd be impossible to bring to a Switch successor, technically at least.
Given it's at near 4K in graphics mode and above 1080p even in performance mode on PS5, I'd imagine that a 30fps port could be done with some judicious trimming of settings, similar to what we saw with games like Dragon Quest 11 or Witcher 3 or Dying Light on the current Switch.

That would certainly be one way for them to squeeze some more money out of it, provided whatever deals they have in place allow for it to be ported elsewhere of course.

It's not a complicated issue at we all know switch is more powerful then 360/ps3 but the hardware felt more like ps3 pro and very much felt last gen. Switch has had 9 years on the market and people still call botw and mario kart 8 the best looking games those were on hardware that was very much 360/ps3 level. Let's compare the most technically advanced open world on Switch vs ps4 they are generation apart easily graphically. 

Neither Mario Kart nor BOTW are among the most advanced games on Switch. They have a nice art style, but there are many games that take better advantage of the hardware's capabilities.

The main problem wthose hose games I they look worse or still in the same ballpark while ps4 games looked they belonged in a new generation 

Same ballpark as what, PS3/360? I can't think of anything on PS3/360 that looked as clean as MK8 does running in 1080p/60fps on Switch, or with as many dynamic effects going on as BOTW.

When I say same ballpark I mean something like ps3 pro that can do 1080p and much better ram set up. We all know wiiu was on par with 360/ps3 heck didn't almost every port run worse then 360 version.

Switch isn't the Wii U though, it's quite a bit more capable. Even a pretty straightforward port like Mario Kart 8 is more than double the resolution on Switch compared to Wii U.

Obviously we said switch is more powerful then then 360/wiiu/ps3 but it's more like wiiu pro and that's why we say it's games just didn't look like they belonged in the ps4 generation graphically. When you look at the things like last of us 2, gow of war, red dead 2, and many other it's looks like a different generation of hardware.

I don't think porting anything is impossible (within reason).  It is always a matter of time, money and sacrifice.  

The problem with Rebirth for the S2 is file size.  It is just under 150 gb.  Square doesn't strike as willing to pay for large carts...  and who wants to download something that takes up 70% of the storage?  

File size is a big question for me on the S2.  256 gb is low, my rig is 4 TB.  

The open world sections of Rebirth I think would tear the S2's bandwidth apart.  So I don't think a straight port would be possible.  It would need additional sacrifices to textures and lighting.  Rebirth was built for 16 gb ram and 448 gb/s.  Going to 12 gb and 112 gb/s is possible but isn't a straight port.

As for silky smooth games, I nominate Crack in Time on the ps3.  The game is stunning.  

I honestly don't feel Switch fits neatly into either the PS3/360 or PS4/XBO generation graphically; it's kind of this weird middle ground.

File size can be drastically reduced in porting; Sniper Elite 4 on PS4 is 35GB, not on Switch it's just 6GB, 1/6th as much while retaining quite good quality.

In terms of bandwidth, that's what settings reductions are for, same as how PS4 to Switch ports like Witcher 3 or Doom Eternal got down from 176GB/s to 25GB/s.