Steven Sinofsky wasn't in charge of Windows Phone when that division decided to have no device upgrade path from WP7 to WP8 (or, for that matter, no upgrade path _or_ application compatibility from WP6.5 to WP7.0).

The person in charge of Windows Phone then was Terry Myerson, who had an entirely different reporting chain to Steve Ballmer from Sinofsky's. Myerson was later promoted to run R&D for all Microsoft operating systems in early 2014. (Yes, that means he was also the person responsible for killing Windows on phones most recently, at least for now.)

The death of XNA also didn't happen under Sinofsky's watch. XNA came out of the Xbox division and was abandoned by the same division, before it was combined with Windows under Myerson. If anything, Myerson kept XNA alive longer by making sure it was still supported on WP8.0 and WP8.1 when he ran Windows Phone, and by ensuring that Windows continued to fund MonoGame as an open-source successor/replacement for XNA.