Earlier in the generation it seemed possible that the Switch might not get to 100M. But at this point 100M is a given. It should pass that milestone by a considerable margin. Still, as others pointed out, Nintendo systems tend to hit a wall, experiencing rapid drops in sales once Nintendo starts to slow down support for the current system and ramp up production and development for its replacement.
Even right now, the upcoming slate of Nintendo-published titles is looking sparse, with a few re-releases/remasters and mid-tier original titles. Almost all of the big guns have already been brought to bear, and that's been kind of the norm for Nintendo systems to have already gotten nearly all of their truly major titles within 3-4 years of release. Sure, we have a BotW sequel coming out, but what else? Metroid Prime 4 will certainly have fans, but Metroid has never been a major blockbuster franchise. I'm talking about the big marquee franchises. Will we see Mario Kart 9? Or another new mainline Mario game? Or another major Pokemon release? Will those be enough if they don't come soon enough?
The Switch is doing fantastic right now, no doubt about it. It should be able to get up to at least 120M units worldwide. But 150M+ like some people think? Unlikely. I still have my doubts that they can keep things going at this level for much longer without some major stimulative factor to help maintain the momentum. If not some huge game releases, then perhaps another major hardware revision and/or a price cut could work, but what kind of impact would those have on an aging system where those things have diminished impact the further you get into a system's life? If they're going to happen, they may need to happen soon. Otherwise, the idea that it has a realistic shot at beating the DS is still looking just as improbable as the idea that it might only sell 100M units. Could that change? Yes. Will it? I have my doubts.
And you know what? It's perfectly fine if it doesn't beat the DS. It's perfectly fine if it "only" sells 120M. That would still be enough to make it the #3 best-selling system ever, roughly tied with where the PS4 is likely to end up. It doesn't need to break records or become the new all-time #1 to be a success. It has more than proven already that it's a commercial success and that the hybrid design is a winning formula for Nintendo.