US did this a long time ago and UA, even when it gate checks, routinely is short-staffed at arrival gate and sends all bags to baggage claim anyway. It's my view that the 20" carry-on which actually fits into the sizer at the gate does fit into the overhead of the 700/900. It's the larger 21" and 22" and even the 20" when it's stretched, which can be jammed into the overhead of the maineline aircraft but exceed the sizer which won't fit.

I have no problem with this. Meds and valuables go in the smaller carry-on.