Scorpio will sell a handful at best, expensive and 4k, which the majority don't have.

No.
Is the Xbox brand fading? Yes, but not faster than the 3DS. The 3DS is on its last legs. After Sun/Moon it might get the 3rd Gen 7 game (Pokémon Eclipse?)
Or Sun/Moon 2, or Diamond/Pearl remakes, but I think they'll hold off on that until after Gen 8 is out on NX, (I assume.)
But that's really all the 3DS has left to look forward to.
Xbox One still has the Slim, Scorpio, Gears of War, And a lot of big 3rd party titles like GTA VI to look forward to. Now granted I don't think it'll finish close to where the 360 ended up and that Microsoft is headed toward another direction with PC, but the Xbox One isn't going away any time soon

Is this a troll thread mate? Xbox one s is probably selling twice as many as ps4 if you look at amazon right now...
Scorpio is gonna dominate next year!

All three home consoles are probably overtracked, the PS4 by far the most, Xbox numbers are harder to sort out since they bundle them with the 360 to make actual figures less accessible.
I don't think the One is overtracked by much though, we've been undershooting NA for some time according to NPD this year, it is irrelevant in Japan, making the numbers there irrelevant in the big picture, and the EU numbers simply aren't big enough to have any huge impact when all is said and done.

But, it's certainly not dying faster than the 3DS, the 3DS is getting old and it belongs in a market that has been cannibalized by mobile devices, home consoles are better off in that regard, the One included.

Didn't Microsoft stop using this argument and replaced it with MAUs cause it sounded "better". Should pretty much indicate that it's now tracking behind or coming close to it.

We don't have official XB1 numbers, but based on estimates, it's very likely it has fallen behind the 360. And there is almost zero chance of XB1 outselling the 3DS. 

Xbox brand been dying for certain people since early 2000s

I think he is talking about launch allinied... so, it's like you compare the 2016 of XBO with the 2013 of 3DS, 2017 of XBO with 2014 of 3DS, 2018 of XBO with 2015 of 3DS ecc.

Also, i think GTA VI will come at the end of gen 8/starting gen 9 as PS5/XBT launch title, so i really don't see it boost even more PS4/XBO, since who wanna buy another GTA should already have the console, and who don't have one will buy the next generation console.

You sure about this lol?

Probably, I've been thinking the Xbox One would wind up with 40-50 million LTD but I think I'll just go with 30-40 million now.

Well, they're not competing and are two completely different products so, I don't see the point in the thread to begin with. You might as well ask if Xbox One is dying faster than Leapfrog or electronic football.

But as a product, the 3DS was much more successful so no, it's not even close.