The tail is odd in that gif don't Pikachu's have different tails depending on their gender as this one has both traits.

It could that they use the same polygonmodel for both genders and then use a separate transparency layer of something similar on the tail for each gender. The gray background does look like this gif may come from something like Maya, a 3D program.

Game hasn't been revealed yet, but it has been announced that Gamefreak is making a core RPG Pokemon title for Switch set to release in 2018 or later. They said this last E3.

That does make sense it does look like part of the mesh is cut possibly due to being on a different layer.

But it does mean Let's Go being a spin-off isn't a possibility like you said it is.

I don't think you have to worry, if history repeats itself.

3DS released in 2012. Pokemon Black and White 2 released in 2012 for DS only.
X and Y released for 3DS in 2013.

Switch released in 2017. Pokemon Sun and Moon 2 released in 2017 for 3DS only. ( i'm calling it 2 to show similarities) 
And now the new pokemon game will only release for Switch in 2018.

Actually the 3DS came out feb/march 2011, So if history were to repeat itself the game this year would still be 3ds only. 

Ah so you believe Let's Go isn't the Pokemon RPG Nintendo's been referring to. That's an odd stance to take given that it lines-up with the 2018 window and is described to be a mainline game. Regardless there's still no reason to think Let's Go is a spin-off. The leaks (which are how we know about the game to begin with) say it's a remake of yellow for Switch thus a mainline game not for 3DS.

I was planning on getting a switch depending on what I saw out of Metroid Prime 4, but a completely redone Gen I is very tempting. Honestly, I haven't played Pokemon since Gen 3 and watched the other gens minimally on youtube just to see if they could catch my nostalgia. I don't know yet if this will push me to a switch, but it just might.

Except Game Freak said that they are done with 3DS