If you can't guess by my avatar image, I really like Dragon Ball. I just saw some images of DBX2 for switch. There was no video, but the images looked pretty good. OK, the first Xenoverse was for PS4, Xbox One, PC, PS3 and 360. I own the PS3 copy of the first Xenoverse. It runs at 30 fps but slows down to around 15 fps when there is 3 vs 3 players and a lot going on. 

Xenoverse 2 is next generation only. PS4, XB1, and PC. It runs at 60 fps. There are more effects as well. Well, it was announced for Switch awhile back. Now that I've seen screenshots, I wonder if the game will be cut down for switch. Like 30 fps and/or less effects. If so, then why was there no PS3/360 versions? I wonder exactly how Switch rates in specs next to PS3/360. 

At this point, how many people still buy new games for the PS360, specially when their performance is going to be compromised? DB Xenoverse 2 will probably be the weakest version, but if the performance doesn't become unplayable, I don't think people will care. Would it be much for the Switch to play the game 720p 60fps with little to no effects while docked and 720p 30fps with barely any effects while undocked?

Well now, your Persona 5 picture is a good example. It just released for PS3, despite being newer than Xenoverse 2. 

from what i understand Switch is roughly halfway in between PS3/360 & PS4/XBO

Xenoverse 2 is really good, I just got it on steam and have sunk in 8 hours and Still have plenty more to go. Hopefully you can get it on switch :)

Switch is a bit stronger than the Wii U, which was already about 50% stronger than the 360. In Docked mode the Switch is actually about twice as strong als a 360, and Switch comes with much more memory than a PS360 console.

So either they where not powerful enough anymore, have not enough memory for the game... or simply because they are end of life and new games for them don't sell well anymore.

Perhaps it will just be cut to 30 fps then. 

Persona 5 started as a PS3 game until Sony asked them to move the developement to the PS4. They just kept the PS3 version around because there was really no problem doing so. It's a turn based JRPG, those are usually not that demanding, at least not as demanding as 3D fighting games. I don't think Xenoverse 2 had PS360 in mind when it's developement started, so they won't bother with a downgraded port. Again, the Switch version will be the weakest one out there, and it's probably only there to test how well an Unreal4 game fits on the system.

I would guess its actually more powerful than 360/PS3 even in portable mode.

Best example I can think of is how Minecraft runs with higher framerate and larger world size than the 360 version even in portable mode. I mean it's Microsoft developing this thing! Numerous PS4/PC ports lined up for Switch suggest the same.

Well obviously the Switch has much better hardware than the PS3/360/, undocked it's still considerably more powerful than them. On top of that it uses 2015 technology and has 4gb of ram compared to 512mb. It also has support for things like Vulkan, OpenGL 4.5 and OpenGL ES etc.

Given the Switch's much more capable hardware there is obviously going to be games that can run on Switch but won't run on PS3/360.