I like the idea and if you recall, google tried that with its unlocked
Nexus One running android "cupcake". It was only sold via web stores in Jan of 2010, After all the support complaints rolled in, they quickly hooked up with carrier stores in July of the same year. Surprise, surprise, people only know how to go to carriers to fix broken phones.
That is its history and the question is, is MS prepared for the support part? Moreover, US carriers have this very ugly side of not supporting a phone's features with bogus technical complaints. AT&T recently tried to bill Apple iPhone owners more to use its Facetime
feature. Which would be a real detriment to the hearing impaired community. Point is, carriers could simply restrict a feature like Cortana and without legal muscle, that shiny new 950 XL
becomes the 950 VCR