At first glance, it's easy to dismiss the Switch as yet another first party-centric Nintendo console as it's so far not been able to get a whole lot of third party multiplatform titles from PS4 and Xbox One. You could say that its because NIntendo chose to make another under-powered console and that they should've made a PS4 killer if they wanted third parties. While that sounds good on paper, many gamers forget, or don't want to admit one thing regarding third party support. Developers, actually don't care about power that much.
Now this doesn't mean power isn't important, it's always important. But ultimately, its actually one of the least important aspects when it comes to developers deciding to support a console. A console's third party support actually has little to do with how powerful it is. It has more to do with audience, user-base, ease of development, and any unique gimmicks or features. Take the Nintendo DS for example, yeah compared to the PSP, it was woefully under-powered. But to third parties, that didn't matter. Why? It sold 150 million units worldwide, a diverse range of consumers buying different kinds of games for it, had a bunch of then innovative tools to play with such as the touch screen, the graphics were good enough for most genres, and it was easy and cheap to develop for. As a result, it received just as good, in several cases better support than the PSP.
Now I know some of you will try and use the Wii as a counter argument, but the Wii had a whole different set of issues regarding third party support. First, the console wasn't developed with HD in mind, which meant it lacked the shaders and development tools needed for HD games, which often meant porting to the Wii required re-developing the game from scratch. The Wii Remote and Nunchuck also had a limited button layout, which meant the control schemes would've had to have been drastically reworked, often giving certain actions to motion controls, which for certain actions and uses, aren't the most precise options available. And even though the Wii sold 100 million systems worldwide, a good majority of those sales came from casuals and non-gamers, who bought it for Wii Sports, Wii Fit, and other similar titles. Which meant more core gamer oriented titles often under preformed, and this affected how developers saw the console.
The Switch by contrast has none of those issues. While its no Xbox One, the Switch is powerful enough that most PS4 and Xbox One content can be ported over with relative ease. Much like the DS, it's easy and low-cost to develop games for thanks to its Nvidia Chip-set and development tools, making it an especially attractive option for indie developers. It has a bunch of neat gimmicks and novelties, but unlike the Wii Remote, the Joy-Con also have the layout of a standard controller, which means they can act as a traditional controller when needed. And even more so than the Wii or DS, the Switch is appealing to a wide variety of consumers and gamers, who are buying a variety of different types of games for it. Not only are casual stuff like Just Dance and 1-2 Switch doing well, but so are Breath of the Wild, NBA 2k, Skyrim, Street Fighter, and a slew of indie titles. Switch owners are buying core games, lots of them, and that's encouraging to third parties.
Point is, the Switch is in the best position to have Nintendo's most reliable third party support in years, despite being less powerful than the contemporaries. Now this doesn't mean every game will get ported over, there are obviously limits to what you can run on a tablet. But if third party sales keep up, we won't be missing as many games as you may think.