Sorry, Johnny, but the reason why US wanted to dump its slots at LGA was because there were a handful of markets from NYC where it still had the size on the "other end" of the route to command sufficient amounts of traffic on the flight. Cities in the Carolinas and Virginia were the few places outside of US hubs where US had strength in NYC.
US pulled out of many of the largest LGA markets in the early 2000s including the Florida market. Florida markets can be low yielding but remember that there are huge numbers of commuters that work in NYC but live elsewhere including in FLA. They don't all pay trash fares.
In fact, DOT data shows that DL's average fares at LGA were slightly higher than US' at DCA, even given the longer distances from NYC to most of the top destinations.
US used the majority of its slots at LGA to fly dozens of times per day to PHL and on turboprop aircraft to cities around NYC, but LGA was not a hub for US as DL has made it. DCA was a hub for US before the slot deal and they have used
NYC is the number one destination for almost every city in the eastern US... despite US' attempts to say that NYC doesn't matter, it is the largest market.
But more specific to this conversation, DL used the much larger number of slots it gained from US compared to what US gained from DL in order to add new flights in markets up and down the east coast, including into key US markets.
AA/Eagle also has flown LGA to a number of key US markets including CLT.
The simple fact is that DL also used its slots to add service into virtually every market from LGA that every other carrier serves from LGA as well in an attempt to provide little room for any carrier to have an advantage over DL in NYC; most of those same markets are served nonstop by UA from EWR as well.
DOT data also shows that US obtained decent average fares in many of the point to point markets it flew with RJs from LGA... the problem was that there were way more slots than US could use.
It is well known that hubs allow carriers to gain greater power to compete in the local market because of the connecting passengers that flow thru the hub.
It is also well established that LGA is the first choice among the NYC airports for every market that can be served from LGA.
While CO/UA has had a dominant position at EWR for years, LGA and JFK have been split among multiple carriers for years.
DL's strategy at NYC was to add a domestic hub at LGA just as B6 has at JFK and UA has at EWR but to use the preferred airport status of LGA to gain a revenue advantage over other carriers and in so doing free up some of its slots at JFK to add flights that could only be flown from JFK.
US' strategy at DCA is similar to DL's at LGA.... the simple difference is that LGA average fares are higher to comparable markets, NYC is a larger market overall, and DL has both longhaul and shorthaul service from two airports while US has only the shorthaul service from DCA.
To argue that DL's service will fail at LGA while US' will succeed at DCA is not only a poor rendering of every fact that both companies presented in arguing for the slot deal but also the realities of each market.
DL is simply a larger airline and better able to compete in a larger market like NYC while US had an established position at DCA on which it could build.
US traded away what it could not use to DL who will gain far more in terms of its ability to advance in a market that was more fragmented than DCA where US already had a decent position but was able to use what DL decided could best be swapped away for more LGA slots.
Since there aren't a whole lot of cities that have nonstop int'l service, I'm not sure what the point is about carrying int'l traffic... it is going to connect on some airline somewhere.
Industry data shows that the carrier that has the largest domestic presence from a city usually gets a higher percentage of the int'l traffic - as long as it is to a region of the world where that carrier competes.
DL is simply a larger airline than US and is heavily concentrated in the eastern US via its hubs at LGA/JFK, DTW, and ATL each of which have international service to multiple continents.