Last I checked I can use iTunes on a Windows PC without issue. I can also use MS Office products on Apple and Chromebooks (via web).

Companies will add their products when and where it makes sense so long as it does not hurt their primary business.

Nintendo will put mobile specific games to generate revenue and entice users to buy their core products... their hardware and software.

You can get iTunes on windows because it sells more iPods, iPads and iPhones, and to increase sales on the iTunes store (iTunes existed as a mac-exclusive product for years before the introduction of the iPod and iTunes store). They only did this to push their own hardware, and their own online store. This is totally different, as Pokemon Go in no way interacts with any Nintendo console or handheld. The two are completely seperate entities.

When it comes to Office being on Macs and Chromebooks, again its mostly to software increase sales, Microsoft is mainly a software company afterall. Microsoft has been fighting for years to make and keep Microsoft Office an industry standard. But Microsoft is a whole different animal than pretty much any company on the planet. It does what it can to put its software in the households and businesses of as many people as possible. I'm not sure if the Xbox division overall has made money yet, and Microsoft has sold over 100 million consoles world-wide. That's mainly because Xbox exists to get Microsoft in your living room, to add another Windows-friendly device to make the Microsoft software eco-system that much more appealing to the average user. If you have an Xbox, you're probably more likely to want to buy a Window's PC as a result, and use Microsoft's stores, and buy MS office, etc.. But let's be clear - Xbox is a small fish in a big pond for Microsoft, MS can continue to lose millions every year if it means its other software endeavours do better as a result because the Xbox division only exists to prop up the other divisions. It's not really a fair comparison to pull in Microsoft because of this.

For a similiar argument to be made for Nintendo, you need to demonstrate how Pokemon Go, or any of it's other forays into moble (past and future) directly leads to more Nintendo hardware sales.  Perhaps that's why we're hearing rumors about the NX having mobile phone connectivity support, Perhaps Nintendo is trying to bridge the gap between mobile and their hardware to make an enhanced experience. But right now, based on the Nintendo products we know of, based on what we know is true, I'm not seeing it at all.