What Diamond said. They have Mario and Zelda but that still leaves quite a few big guns (speaking both commercially and critically here.)
The Switch will get two generations of Pokemon (plus a bunch of spinoffs), a (unified) Smash, a new Metroid Prime, Bayo 3, Animal Crossing, Kirby, Yoshi, and honestly, probably another Mario and Zelda. The last Nintendo console success, the Wii, got two mainline games for each. There will be multiple Fire Emblem games, which is now popular in the west post-Awakening. Another New Super Mario Bros., despite the series rocky quality, would sell gangbusters.
And if they wanted, they could dredge up any of the following to pad things out: Sin & Punishment, Luigi's Mansion, Kid Icarus, Donkey Kong Country, F-Zero, Wario Land/Ware, Fatal Frame, Star Fox, Excite Bike/Truck, Custom Robo, Pikmin, Mario Party, etc. Let's also not forget that Nintendo has launched two successful all-new IPs in the last few years: Splatoon and ARMS. So there's hope there, too.
Will the third-party ports last? Maybe, maybe not. The Wii couldn't handle PS360 games to the point where it was more economical to make completely new games (Umbrella Chronicles, Dead Rising, TatsuCap, etc.) The fact that the Switch is getting Doom, etc. at all is cause for optimism. Not to mention the SMT5 exclusive lockdown.
I've always maintained that the success of the Switch could be foreseen by combining the 3DS and Wii U libraries and asking prospective console buyers if that would be good enough to buy a single system for.
Will the Switch's second year (and after) suck? Maybe. Based on history, I'd probably bet no, though. When a Nintendo system flops, it kinda flops out of the gate, and vice-versa for their successful ones. The only outlier is the 3DS, but the measures to turn around that sinking ship were unusually drastic (and not applicable to the Switch's situation, since, y'know, the Switch is the fastest selling console of all time in the United States.)
Yes, the Switch had a strong start, but that doesn't mean there's nowhere to go but down.