It would be interesting to know what Microsoft were thinking in 2013. Did they say anything back then about future upgrade-ability? Or maybe they had other priorities? I'm not suggesting they did know that the phones wouldn't be upgradeable, but if that had been the case, that's extremely short-sighted.
iOS9 is supported on a 4S, which launched at the end of 2011. Surely getting Win10 to run on a 3-year old WinPhone is a pretty low bar to set?