I predict Nintendo to make a successor to the Switch after 5 years. I can't see the tech lasting that long with too many newer systems around the corner.
X1 will probably continue to sell. X2 will most likely be part of the X1 family and will continue to add to the sales of X1. Plus than you will have Halo 7 etc..
Doesnt matter if gamers already own an Xbox for Halo. Halo 6 will sell more systems. It will be heavily marketed and bundled plus you also have collectors and gamers that buy more than one console etc. Halo 5 made 500m on launch. Expect similar results with 6 etc.
The tech in the 3DS was even older for its time and 4 newer systems have come out since it's launch, yet it's in Year 7. I see no reason why Switch would have a shorter lifespan.
Halo 5 didn't even boost sales much long term, it did well enough in the holidays but a couple of months later Xbone was back to selling sub-80k weekly. If 5 couldn't change Xbone's fortunes, 6 won't either.
3DS is a different system to Switch. As a Console Hybrid its tech will be extremly old in 5 years (it already is outdated). Maybe for the portable market it will be fine however home console user will want something new which is one major selling point on the Switch. Also remember the 3DS has no competition while Switch does.
Halo 5 sold very well. It decline towards the end however take in consideration that Halo 5 isnt considered a great Halo game in the Halo Universe. If Halo 6 turns out to be amazing than theres your Pokemon counter. Plus there will be a Halo 7 probably in the next 4 years.
This is not including any other Major X1 titles releasing that might be huge hits etc in the next few years. I doubt MS is just going to sit there and let there system decline in sales when they have Scorpio and possibly X2 sharing the same eco system coming out.
Plus MS isnt giving figures for there console. We wont know how well its selling apart from speculations and assumptions.