> If they hadn't merged with Boeing then it would have been with some other large aerospace or defense company like Raytheon or Lockheed Martin

Which is kind of the idea. You then have Raytheon or Lockheed serving the part of the deep pocket who takes possession of their infrastructure instead of handing it to their only competitor. That company may then notice that they're in a position to take a piece of the huge commercial airliner market if they would make an R&D investment into the thing they just bought, and unlike MDD, they have the resources to do that.

Conversely, the buyer isn't interested in the civilian market at all and only buys them for the military assets and sells the civilian infrastructure at fire sale prices to whoever will take it. That still isn't allowed to be Boeing so you now have someone getting a couple billion dollars worth of infrastructure for 10% of its original creation cost, and that make them a viable company because they're starting off ahead by a large amount of capital infrastructure they didn't have to pay the normal price to get.