As someone who also developed on windows phone I really disagree with this portrayal. Microsoft did not change the stack on account of "getting bored" with the tech.

Looking back on it, they took on an extremely difficult problem having a completely new marketplace and platform against competitors with tremendous amount of pre-existing traction.

There were clearly some pivots towards universal windows compatibility, and possibility of android compatibility as well. Nobody argues there's a ton of tactical failures and the net result was a loss, but I wouldnt conflate the mobile endeavor with something half-baked nor disregard to developers