"There is only one exception to everything I wrote above:
Nintendo continue to launch revisions under the Switch moniker for years to come that go beyond the usual lifespan of a console (15-20years) without considering it a jump in generation or a new system."

This is a big reason why is still can sell, but I do think you're not giving enough credit to the advantages the Switch has in circumstance over the others, as if it has none.

That's why everybody is considering a switch mini. 4.5 to 5"screen+retractable joycon,ditch the dock, the grip and sell in the 150 to 200$ range.
Technology will eventually make it possible to happen. I'd bet 2019, just on 3ds death.

Not a chance it does ds numbers. No handheld ever will again.

I think it's easy to forget that the D's launched in 2004. A good 3 years before the iPhone launched spearheading the smartphone market. And as such a good 6 years before the smartphone market took hold.

The simple truth is that today it makes more sense buying a $50 tablet or if you are feeling generous a $90 tablet for your child than buying them a handheld console. That's the simple reality we live in today.

As long as the smartphone/tablet market exists, the dedicated handheld gaming console cannot reach it's peaks of old.

What the NS has going for it though is that it's a hybrid. Individually it won't sell as well as a PS4 or a 3DS, but as a hybrid it could push around 60-70M units by the time it's all said and done to what at the time would be around 100-110M.

At some point the NS would cost as little as $199 or even $149.... But at that point the PS4 could cost as little as $149/$120.

I could definitely see the Nintendo Switch outselling the Wii, as it gets multiple hardware revisions and a longer life on shelves (along with all the other stuff you mentioned).

The only thing really stopping the NS from outselling the DS (long term) is that Nintendo will probably release a backwards compatible successor (maybe in 2024?) hamstringing its potential lifetime sales (maybe ~110m lifetime?).

And, obviously the always unknown factor of other new cool devices might switch things up as well, providing something people might buy in lieu of a Nintendo Switch (though I think this is rather unlikely seeing how stagnant things have been the past 5 years, sans the NS of course :P ).

No console will ever again.

Is anyone really expecting DS numbers?

It's a Nintendo console we are about here. 

People tend to get carried away. 

Nintendo can sell multiple Switches because there will be multiple versions of Switch as time goes on. I agree that parents would be less inclined to buy multiple Switches for the reasons you stated. Switch is more amenable to being "shared" as a system than the DS/3DS are, that said Nintendo can bend the rules there and offer more frequent hardware refreshes of Switch, thus creating other incentive to have multiple units in a house, the same way Apple and other mobile makers get such high sales ... you think if they just release one iPhone every 5 years they'd have even 1/2 the sales? Nintendo will adopt that upgrade process to a degree IMO. You'll have a higher end Switch model in about 3 years IMO. 

That said, any hardware is in tough if the goal is PS2/DS or bust. That may very well never happen again not in the way it did then. Even the PS2, needed like 10+ years to hit those sales, DS did it much less time. There's a good chance you'll never see anything like the DS again, not from Nintendo/Sony/MS. 

I would say that a Switch could be appealing to casual audiences, specifically parents with kids. Recently, I always see parents needing to buy a distraction for their kids so they can play games on long drives and not be obnoxious. The number one device I see around myself that parents buy for young kids is an iPad. The Switch resembles closely to an iPhone/iPad sort of device, and the games on it would intrigue kids like they are intrigued by home consoles and handhelds. On top of that, $300 competes very well against expensive tablets like the iPad.

Personally I think if it can continue the core gamer/family friendly gaming device, this thing could possibly (personally I say easily) beat the Wii. The PS2/DS sales I think are actually achievable, but not likely. For now I think Nintendo is in a good place and keep the momentum of the Switch at full speed.

The problem there is like you said ... most parents already have a tablet for their kids. 

Switch is going to have to make hay with an older audience with more disposable income who reeeeeally want more complex game experiences than a tablet can offer, I think the kids market is less reliable for Nintendo going forward because mobile appeals so much to kids, and parents love it too. 

My parents would've been over the moon if NES and Super NES games cost $0 instead of $60-$70 a pop, so it's not hard to see why parents love just giving their kids a tablet and letting them sit immersed in that for hours and hours on end. 

$300 can buy you a pretty high end Android tablet too ... it doesn't neccessarily have to be an iPad. Even $150 really, you can get yourself a solid tablet for the kiddies that will play cartoons, thousands of games, take photos, etc. etc. etc. 

Why is it okay for the PS4 to beat the PS2 in sales but not the Switch get close to the DS? Do I think either will reach those sales, no! But the system just launched and demand is there, so let's wait and see what it does.