by the way, 4.5 block hour days are really not all that impressive, but if they can do this daily, it will be already much better than what ARJ21 is capable of. It is very normal for an airline to introduce a new type and fly it between their biggest stations, so that if something break down, they can fix it easily. They can also put passengers on a different aircraft or a different flight in the world case scenario. Long term, MU is unlikely to operate C919 on a route where they typically use wide body aircraft.