Windows Phone is growing, albeit slowly, but it is growing, and Windows Phone 8.1 is actually quite nice. When you buy a Windows phone (Nokia) you actually get a good quality device. I have never been crazy about Samsung devices-- they feel cheap but that is my opinion. Others don't feel that way, so the future will be interesting if they proceed down the Tizen path.
My guess is that Samsung will push Tizen on the lower end, low-spec phones that it can run well on, but Android doesn't. Samsung will also pitch Tizen to mobile carriers in the US as a platform they can easily lock down like they did in the "good ole days" of feature phones. Something they can't do as much with Apple and Google.
All of this sucks for consumers, but hey it's the US carrier market. It's still the Wild West here. This country is about 15-20 years behind the EU in consumer protection and regulation of the mobile phone industry.