It seems most airlines put brand new aircraft on shorter routes upon first delivery - I guess to work out the kinks and train the crew. The 77A was definitely in this situation. I think once that is settled, they will do some shorthaul to the routes with the most demand (e.g. SGN).

I think contractually, they have the right to substitute aircraft. So, it is really a customer satisfaction thing. I doubt they do it purosely as a "bate" as OP speculated - it is a pain when they swap from a bigger plane to a smaller plane.

I do find them better with IRROP handling but definitely not generous with compensation. Would you rather to have the UA way of "so what we screwed up, here is 10,000 miles or $400 voucher and we will crew up again"