Well, after all the movie-plot theories, mechanical possibilities, crew conspiracy/suicide suggestions; after all the pings and turns, descents and climbs, and despite volumuous discussions of switch positions, radio phraseology, and aircraft systems, we're still left with nothing more than what we started with:
An extremely reliable airplane, maintained by a reputable airline, flown by a professional, qualified crew, on a normal route with no adverse weather.... that inexplicably doesn't make it to its destination and disappears.
Since everyone has their own opinion as for why, I'll offer mine and make this my final post on the thread.
A security failure at some point resulted in a person or persons, as yet unknown, interfering with the flight crew's performance of their duties, for the purpose of terrorism. The crew died trying to thwart that effort, resulting in the tragic loss of the aircraft and all on board. As for why no group has yet claimed responsibility, they failed. Whatever building or major city was their target is still intact. No terror group will advertise their failures.