You are absolutely right, AANOTOK, that we will never know how AA and US would have fared as standalones now that the decision to be made has been made.
It is also true that the creditors had to face the very real prospect that AA could have been even further marginalized in key markets, not just by DL but by other carriers as well.
However, it is equally true based on the past that DL has been able to expand in both AA and US markets with very little resistance over the past 5 years and there are no indications that DL is changing its strategy. In fact, the number of seats in nearly all AA/US hubs and key markets is increasing this summer compared to last even though DL's system capacity is expected to be down; DL knew full well that the merger was coming and continues to build its presence in key AA/US markets so as to mute the effects of the merger.
Even if AA/US goes on the aggressive against DL at LGA, DL still has established itself with seat shares exceeding 1/3 of the total seats in the market in LGA-MIA and since DL has no hub on the other end, the number of seats DL sells to local passengers is much higher. DL is very close to seat parity with US in LGA-PIT right now. DL continues to grow LAX while AA and UA are both shrinking there. DL is once again the largest carrier at JFK this summer in terms of both seats and flights. (and Bears, DL's international presence at JFK is twice the size of AA's - and DL has its own int'l presence at EWR that will be enhanced in both places by Virgin plus Skyteam.... .let's not forget that Iberia continues to shrink - a couple thousand more employees will be laid off in the very near future).
DL's position at JFK, LGA, and DCA relative to the new AA at the same airports will continue to be a key strategic determiner of success on the east coast.
Even though DL is the largest carrier at both LGA and JFK (and the new AA will not surpass DL in those markets), DL does 47% of the slots at JFK while US has 55% of slots at DCA. The feds will likely argue that they cannot allow US/AA to exceed US' current position of 55% of slots at DCA; the only slot controlled airport where there is a higher percentage of slots than that is EWR where UA now has about 75% of the slots.
US could agree with the feds and be forced to divest nearly all of the slots it gains from AA at DCA or it can accept a higher percentage of slots at DCA. But if AA/US ends up with 65% of the slots at DCA, DL could very well argue that it should too be allowed to acquire 65% of the slots at either JFK or LGA, or both, since the feds would have allowed the new AA plus UA to have that high of percentages of slots at DCA.
At that point, then DL could go after B6 which has a pretty low market cap and at the same time gain a whole lot more market access not only in NYC but also in BOS, DCA, Florida, and to the Caribbean and Latin America.
Even if DL doesn't do anything to add slots, slot divestitures by AA/US at DCA will very likely exceed whatever revenue synergies they would have achieved at LGA and JFK where both are fairly complementary.
There really is no telling where the next strategic moves will be but history points rather strongly to the fact that DL has given nothing up to AA, UA, or US over the past decade and there is no reason to think DL is going to let that track record change - of course if history is any indication.
DL could also shift its strategic focus west with an acquisition there that would make it much harder for AA/US to gain synergies. If PHX shrinks, DL at SLC is likely to be one of the top hubs to benefit.
DL could also continue to focus on building its presence in key AA/US international markets as it is doing in the UK, Mexico, and Brazil with its partial ownership of Virgin Atlantic, AeroMexico, and Gol.
It is one thing to argue that the merger MIGHT slow the assaults that AA and US have both sustained on their networks from other carriers; it is quite another to suggest that AA/US will start turning back the tide esp. against DL which has demonstrated an ability to find opportunities where others have not. It is still a half year or more from merger approval and other carriers including DL are expanding in key AA/US markets.
BTW, my market specific responses to the issues raised in the article and by others in the post are about the same length as the article cited by the OP. No copying and pasting of articles (esp. w/o comment) will go along ways to limit what I can respond to.