If Windows S turns out to be "guaranteed safe", it'll be a breakthrough for folks like your wife.
However, it's still Windows, and it's still just an operating system, not a panacea. Color me skeptical that "guaranteed safe" will really apply to WinS or any operating system. Microsoft's aim is not safety, per se. It's to drive people to the Windows Store. They see the Store as their future revenue stream, as their historical ones dry up.
You’re probably right about the motivation—shepharding users to the Windows Store.
But, as I was saying to one of my boys the other day, it’s surprising how much “street smarts” someone using regular Windoze needs to not click on some malware and accidentally screw up their computer. It’s second nature to anyone who’s been using computers for a while, but for those who need an information appliance, it’s a problem. So, if “S” can deliver that, it would be a good thing.