Never said it would stop selling, but sales usually go down by alot after a successor releases. PS2 was the main exception to that rule, it continued selling for 7 years after PS3 releases, and managed to sell an impressive 40m+ after PS3 released. DS by comparison only sold about 10m after 3DS released. PS3 sold about 6m after PS4 released. I would expect Switch sales after Switch 2 releases to fall somewhere between those two, so between 6 and 10m. 

Switch is classified as an 8th gen console according to Wikipedia at least:

https://en.wikipedia.org/wiki/Eighth_generation_of_video_game_consoles

I agree with that personally, Switch released March 2017, about 3 years and 8 months before Xbox Series and PS5 will likely release in November 2020. I can't classify it in the same generation as PS5 and XS bearing that in mind, and we've never had a console generation where there was only a single console, so I disagree with putting Switch in it's own 9th generation and having PS5 and XS be 10th gen consoles. 

Did it take like 4 years for us to see another system after the dreamcast?

Your second paragraph is priceless because even if the part about coming in the middle was true or the last part had any relevance to generations, it would mean the dreamcast was 5th gen. Both switch and dreamcast released 4 years after the start of previous gen. A little earlier than usual but not in the middle of anything. Since time of release is all that matters in this subject, that makes the dreamcast a 6th gen, switch a 9th gen, and ps5/xsx/switch 2 10th gen. How long existing consoles over/understay their welcome is completely irrelevant.

Technically, switch is a better Wii U, so i would consider it an 8th gen console that doesn't share sales with the Wii U. We could also act like Wii U never happened.

This.  People are constantly overestimating Nintendo's legs for their consoles, while underestimating Sony's.  I also remember the people who said the PS4 was never going to beat the Wii, even though 2 out of 3 of Sony's previous systems did.  And the one that didn't came in a pretty close 2nd in that same gen with the Wii, 101.6M vs 87.4M.  And that was the gen they utterly screwed up.  Then, we had it won't hit 110M, which it will by the end of this quarter.  And now, it's "it won't hit 120M," even though it is a mere 12M away from that number, now. 

Now, I can definitely see Switch hitting 100M, give or take 10M.  But, it's not going to hit 125M, like the PS4 will most likely do.  I mean, the PS4 looks to be on track to sell 120M even without a price cut.  Give it one or two more, I can definitely see it hitting 125M-130M. 

I would just like to know what is going to keep the Switch selling ~20M every year for the next 4 years to get it to that number.  By 2021, all of Nintendo's big hitters will be out.  It will be receiving fewer AA and AAA ports thanks to those teams focusing on the newer systems.  Speaking of which, the novelty of being the new toy on the block will have completely worn off, as well.  And it will have already had a $199 SKU for over a year, pushing sales for those not willing to pay $299, so a price cut will have less of an impact by then than it would for the PS4.

This.  People are constantly overestimating Nintendo's legs for their consoles, while underestimating Sony's.  I also remember the people who said the PS4 was never going to beat the Wii, even though 2 out of 3 of Sony's previous systems did.  And the one that didn't came in a pretty close 2nd in that same gen with the Wii, 101.6M vs 87.4M.  And that was the gen they utterly screwed up.  Then, we had it won't hit 110M, which it will by the end of this quarter.  And now, it's "it won't hit 120M," even though it is a mere 12M away from that number, now. 

Now, I can definitely see Switch hitting 100M, give or take 10M.  But, it's not going to hit 125M, like the PS4 will most likely do.  I mean, the PS4 looks to be on track to sell 120M even without a price cut.  Give it one or two more, I can definitely see it hitting 125M-130M. 

I would just like to know what is going to keep the Switch selling ~20M every year for the next 4 years to get it to that number.  By 2021, all of Nintendo's big hitters will be out.  It will be receiving fewer AA and AAA ports thanks to those teams focusing on the newer systems.  Speaking of which, the novelty of being the new toy on the block will have completely worn off, as well.  And it will have already had a $199 SKU for over a year, pushing sales for those not willing to pay $299, so a price cut will have less of an impact by then than it would for the PS4.

You simply made up a gen for Switch to exist in all by itself. How does that make any sense? Switch is either part of XB1/PS4/WiiU gen or its a part of XbSeries/PS5 gen. Its not its own gen just because it launched in the middle of a generation. It came out pretty much exactly in the middle of gen 8, will likely spend the majority of its lifetime before being replaced with only gen 8 systems on the market. So it is more current gen than PS5/XBSeries gen. And its absurd to put it in its own gen just because it didn't come out at the same time as other systems.

And I don't think you understand my sentence about the Dreamcast, even though I clearly stated it: DID THE DREAMCAST GET A GEN ALL TO ITSELF? Answer: no. Why would the Switch? Dreamcast came out in the late-middle part of a generation (3 years after launch of N64, 2 years before GC/Xbox, with PS/PS2 launching early both those gens) as the early successor for Sega's failed Saturn about 4 years after it (like the Switch launched about 4 years after Wii U). So Dreamcast was either gonna be in the Saturn/PS1/N64 generation or the Xbox/GC/PS2 generation, not a generation of its own, as my question points out. So yes, very relevant, truthful, and accurate. Thanks.

And you still don't explain why a Switch 2 won't be part of its own gen 11 coming out years after your gen 10 even though you gave Switch its own gen for coming out years after gen 8. You have no consistency with future declaration of gens (Switch 2) or past accepted definitions of gens (Dreamcast).