Switch is going to hit 50m milestone at the end of 2019.

Switch still has a lot of cards to play in the upcoming years

Animal Crossing - 15m seller
Zelda BOTW 2 - 15-20m seller
Next 3D Mario - 15m seller
Next Pokemon remake - 10-12m seller
Next mainline Pokemon - 15m seller
Price cut
Another SKU
Evergreen titles discount/promotion.

Gameboy sales may have slowed down, but that wasn't the reason for the Virtual Boy. That was more of a pet project of Yamauchi and Yokoi, the former also not overly fond of the Gameboy the latter invented (and even filed a patent for it in 1982). Yamauchi continued to defend the Virtual Boy long after it's complete failure was clear while it''s failure drove Yokoi to leave the company in 1996.

By contrast, Nintendo released some new versions of the Gameboy (Play it Loud, followed by the Pocket and finally the Color) to keep the sales of the Gameboy flowing. While Pokemon certainly gave the Gameboy a second wind, it would still have been supported for such a long time without it and if the Virtual Boy would have been a success. Would it have been as successful? Probably not, but that's not the point of the question, which was only about the length of support.

About the bolded part: If that was true, then CoD, FiFA, and all the other annualized games would also just push a console's sales once during it's lifetime and not every single year. It will certainly do less than BotW did back then, but it will still push hardware.

And that's leaving out some smaller IP like a new Paper Mario or a second entry in some series like Fire Emblem which often do have multiple entries on a console. And the Switch is simply too big for publishers to ignore by now, so new third party games will come for sure, and this at least as long as PS4/XBO will get ports.

AAA2D Mario - 15-20 m seller

Tomadachi - 5-10 m

PS4 - 130m
Switch - 110m

I think Switch will sell (very) well but like someone posted, new games won't increase that much hardware sells if directed toward the same population. Like The last of Us II, FFVII and (even if not exclusive) Cyberpunk won't make Ps4 sell 20 millions in 2020.

With the new gen rolling in, I think it will be simply a matter of the Switch having to give way to the inevitable Switch 2, before it can catch up.

Switch sales have not even close to peaked.  Take next year for example.  What else are people going to buy?  The Switch is the only new viable product on the market.

On top of that most small-medium size developers are developing Switch games right now.  They can save on development costs if they make a Switch game and they know Switch has a huge (and growing) install base.  These are the same types of devs that made games for the 3DS and Vita.  They are there every generation.  We are still mostly waiting for them to make their appearance on Switch, but they are coming.

Switch sales are still in slow mode.  But the caterpillar is just about ready to come out of it's cocoon.  A big flood of third party games is coming and first party support will continue strong as always.  At the same time Switch has no viable competition until the end of next year.  Expect Switch sales to continue to accelerate.  Switch still has a lot more life left in it.  We've barely just scratched the surface of what Switch will become.

PS4 and XB1 are nearly at the end of their console cycle with the announcement of the next-gen successors while Nintendo Switch isn't even entering the third year on the market. 

Yearly triple-A blockbusters like COD, FIFA, Madden, and NBA are the main force that drove the sales of those two consoles. Of course, it will reach a saturation point where it couldn't move more hardware than before. There were already 6-7 installments of those franchises on the system that makes the growth of the new potential buyer decline.

COD IW, COD BLOPS3, COD BLOPS4, COD WW2, COD AW, and COD MW. 
FIFA 14, 15, 16, 17, 18, 19, and 20. 

We cannot use the argument of the initial purchasers are the one who's going to buy the next game for the switch game yet, it could be the new entry point for a lot of users and etc because the system hasn't yet reached the maturity point. The market trajectory indicates there is a lot of potential rooms for Nintendo Switch in upcoming years in terms of hardware and software sales. 

I wouldn't say they're slow, but 2020 will be Switch's best year hardware wise. The Switch Lite is now out, Pokemon Sword and Shield just got released, Animal Crossing: New Horizons launches March 20. I don't think the momentum will hold, though. I know people say that Switch is in a far different market than Xbox and PlayStation, but their successors will eat some of the Switch sales I'm thinking.

And PS4, XBO and entry level PCs will be other viable products on the market for at least the next three years for that kind of games.

So why should these small-medium size developers limit their audience to the Switch hardware base instead of going multiplatform for these games and offer them for a much bigger cumulative hardware base (Switch + base PS4 + PS4 Pro + BC PS5 + Xbox One S + Xbox One X + BC Scarlett + PC)?