I think that LGA and BOS were supposed to get the "DCA-re-do" but were put on hold for lack of equipment. Also, I think this strategy was a Siegel move.
LGA, yes, but as you said there were no planes. Then the talk was that PIT would shrink some more and use those planes for LGA, but that never happened. With the -190's, though, it should be possible. And as I've said, they have almost twice as many slots at LGA as AA or DL, so there's no reason why they shouldn't dominate. (At DCA, they've got over 3x as many slots as AA or DL.) And I'm not sure if you mean to say that this is a bad idea by calling it a Siegel move; I think this is one area where Siegel was dead right.
BOS, no, Siegel didn't plan on expanding there, he was happy with being #3, though God only knows why. (Though US certainly should expand at BOS--they've had over 200 flights there before and were by far the largest carrier. And it's a bit funny--AA and DL have been shrinking at BOS even faster than US, so US is now #1 in seats and #2 in pax, without even trying.) But as BOS isn't slot restricted, US doesn't have an inherent advantage that it can fall back on like it does at LGA. So if they consider to just sit on their behinds and stagnate, someone else will snap up the opportunities that are US's for the taking. And to think that for several years, US had by far the nicest terminal at BOS; now DL's got an even nicer one. US is just lucky that DL hasn't been using it to expand.
Since US has a far bigger advantage at DCA than LGA/BOS, I don't think US will be able to reenter the LGA/BOS-Florida markets, or to fly from LGA/BOS to other airlines hubs, with maybe one or two exceptions. But that still leaves dozens of markets for the taking. I mentioned some of the LGA markets above. Similarly, US should (re-)enter BOS-RDU/GSO/CHS/BNA/MCI/CMH and so on, not to mention (re-)entering the BOS transatlantic market, which is the one place that AA is expanding at BOS. The only good thing they've done at BOS in recent years is the Caribbean expansion, but even that's not as big this year as it was last year due to the lack of planes.
Of course, I don't think Tempe will be expanding at LGA and certainly not at BOS, so I guess it doesn't matter.
🙁