As a data point, I can add that I bought an HTC HD2 back in December 2009. It had come out a couple of months before, and the hardware was very nice - it stayed at the top until this year. However the software was Win Mobile 6.5; it was occasionally updated, but the updates page went away (in a 404 way) back in October 2011 or so.

So, about two years, and gone. Not quite fly-by-night, but not too satisfactory.

(Workaround: using the original OS as bootloader to an Android from XDA-developers ...)