A competition will at least ensure that pricing stays in check. If it is clear from the onset that this is a single-source contract Boeing will try to recoupd all its losses made with the KC-46 so far with follow-on orders.
The USAF could single-source the purchase to Boeing for more KC-46A and just set the price they're willing to pay and set it as a fixed-price contract like the original.
Every follow-on order the USAF has placed with Boeing has been under the existing fixed-price contract and I don't see a reason why the USAF cannot extend the total possible buy beyond the current 179 frames at the same prices.
I find it unlikely that Boeing would tell the USAF to pound sand and refuse to bid considering how deep they are in on the program and every additional frame they sell will help generate future income from spares and block upgrades.
That assums that
a) The frame contract for the 179 KC-46A allows for more to be purchased [over and above the 179] under the same contract. I´m not privy to the contract, I´ve to pound sand on this. However, based on public procurement contracts I´ve seen and bid for the 179 are rather to be considered as cieling, not as moveable barrier. On service contracts possibly, on product contracts rather unusual.
b) Even if the USAF sets a price they are willing to pay and want to set it up as a fixed price contract: there´s always the other party which needs to accept such an order...
c) We don´t know if the current fixed price contract actually covers (production) costs for Boeing. They might decided that it is not worth incurring further losses in the hope of winning support contracts down the line.
d) It assumes that the KC-Y contract specifies a frame very similar to the KC-46A. If this isn´t the case - and it looks as if the KC-Y is more a DC-10 sized frame - if will be hard to bid the KC-46A in anyway.
Flown: A319/320/321,A332/3,A343/346, A359, A380,AT4,AT7,B712, B732/3/4/5/7/8/9,B742/4,B752/3, B762/763,B772/77W,CR2/7/9/K,ER3/4,E70/75/90/95, F50/70/100,M11,L15,SF3,S20, AR8/1, 142/143,... 330.860 miles and counting.