I think cortana will be interesting. We will see, but its not impossible, they just need to make it a wash, and they're not that far off right now.
MS surpasses Google in productivity software, desktop operating systems, design, online file storage, social media integration, and hardware. That's just off the top of my head. Weird comment.
Sure, Cortana might be interesting, but Microsoft is just too slow. Sure they'll release something with Windows Phone 8.1 in the US, but it'll take forever till it's available broadly and till they integrate it into Windows as well.
And as interesting as Cortana is, it's probably independent of their voice recognition technology (unless it's personalized and learns from the specific user). They can already change the TellMe service without an update to the OS and they've done so not long ago.
Of course Microsoft is still big with Window, Office, and Enterprise, and they have lots of great products, but while Google will keep gnawing away at them in all fields they are operating in, I don't really see Microsoft outdoing Google in any area Google's successful in. If both entered a new segment at the same time, I wouldn't bet on Microsoft. Google has the better brand, more mindshare, attracts better talent, and has an unstoppable business model.
Microsoft Research has probably worked on voice recognition for decades. Google swoops in and now they are better at it. Lots of companies such as Navteq have done maps, Google swooped in, and they are now better at it. Google has won mobile and Google might soon be winning wearables. Google Now may become huge and it lends itself very well to Google Glass and Google's smart watch, and there's basically no way MS can match that.
I think Skype might be on the way out too. Skype may have the users (for now), but people don't actually seem to use it that much. They aren't running Skype on their phones, but WhatsApp. Google can swoop in with Hangouts and kill it without spending 8.5 Billions. Integrating Hangouts into Android is a big deal. While Microsoft tries to fix the mess that is Skype and tries to tie it to Lync, Google will just improve Hangouts and add features. I have more faith in Google improving Hangouts than I do in MS improving Skype.
They've even had some success attacking Office. Google Docs is huge among students and in lots of other places, whereas people are relatively unaware of Office Web Apps.
And since Google keeps giving away their software and selling their devices nearly at cost, they are only going to have more success.