In comparison, if WinPho fails its only a tiny part of MS business. The only company depending on WinPho is Nokia (& even they could knock out an Android phone pretty quick if needed).
I think that this is an understatement, to say that it is simply a "tiny part" of MS. As Nintendo and Sega can attest to, a game console is not guaranteed from one generation to the next. I don't doubt that the next Xbox will sell like crazy without issue, it might be foolish to say it is a 100% lock. We're already seeing tablets eat away at the PC market, and we haven't seen Windows 8 taking up enough of those sales to justify the losses in PC sales, so there is arguably some negative long-term growth there as well. After that, you have Office. That will have little trouble because it is so entrenched in businesses.
However, we can see that the future is in touchscreen devices, when it comes to casual consumers. If tablets are not selling well enough with Windows and Windows Phone fails, then you end up with a situation like the company formerly known as RIM had--you're surviving on the kindness of businesses, and if they ever realize they can survive without you, it's a big problem.
That's the overstated negative version of what you said, though. Losing out with Windows Phone might not kill the company, or even do major harm, but it would be a VERY bad thing to have the Zune fail, the Kin fail, Windows Phone fail, and have people (unfairly) bashing Windows 8. It could then be argued that they've had 4 straight failures in products, and it would probably mean the Surface failed as well, which would say that they would likely avoid the hardware business for good, and then stick to just a company of Windows, Office, and Xbox, with little innovation.