Several things:
1. Despite Parker's insistence that the merger has to happen yesterday, there's no fire. If it's a good idea today, it will be a good idea 12 months or 24 months from now. Neither of the other mergers (UA-CO or DL-NW) happened while they were in Ch 11 - even DL-NW was about a year after they emerged from bankruptcy and UA waited even longer before marrying CO.
2. Even if DL is a logical purchaser of US, there would be huge divestments of assets, like the CLT hub - there's no way one airline controls both ATL and CLT. That would permit AA to get the CLT hub everyone insists is the big hole in AA's system that US can fill (SE connecting hub). DL-US would be forced to divest a lot of DCA and a fair portion of the LGA slots, including the shuttle. So what would DL gain from buying US? PHX and PHL? Maybe. I don't see DL chomping at the bit to spend that kind of money to gain those two hubs.
3. UA is a logical purchaser of US, but UA has its hands full for a while trying to integrate its last merger and thus there's no way that US would be hitched to UA within a year or two (see #1 above).
AA has survived for 15 years without a SE connecting hub (since the pull-downs of RDU and BNA) and can easily survive another year or two before tying the knot with US.