If they had to use another engine, they would have to re design as recode most if not all of the game from the ground up.
I don't see that as a posibility, given the compressed timetable that they are contracted for by EA.
To sum up, its only going to get worse. They started with the wrong tools and workflows and 4 years on they are still trying to catch up.
Who knows, maybe Vicarious Visions will completely take over Destiny development and make something of it.
Y'know actually bake a cake intead of just dropping the eggs and then stepping on them