I don't work for WN. I don't work for any airline.
You're not thinking about it from an antitrust law perspective - but I am. The antitrust harm in the proposed DL/US deal is the increased market concentration of DL and US at LGA and DCA, respectively. The feds proposed a remedy and DL and US should have accepted it in a heartbeat. Instead, they arrogantly rejected the feds' position and devised a scheme to divest fewer slots and in such a way as to attempt to prevent WN from getting them. The feds rightly rejected the alternative remedy. Of course slots can be bought and sold - but the Clayton Act prohibits mergers and asset acquisitions where the feds believe the acquirer is gaining too much market power. That WN might convince others to part with their slots at DCA and LGA doesn't change the fact that the feds have determined that DL and US want to exercise too much market power.
The rest of your arguments are based on emotions and not on the market power that DL and US are attempting to gain at LGA and DCA. What other airlines get to do without government interference has nothing to do with the decision.
Excellent analysis. Time for A/T to dust off their capes once again.