It's an interesting build up, but risky...
- While some consider LGA the preferred airport for NYC, I'm not so sure that's a universal opinion.
Having the rail transit options into Manhattan at EWR and even JFK make a difference to many, and there's nothing comparable at LGA (unless the on-again-off-again water taxi is back in service from the MAT).
- The EWR hub probably works better for UACO because they not only can carry day-trippers heading into Manhattan, but flow traffic on top of that, and the availability of an airside connection for those changing terminals.
DL's challenge is that they have to not only manage the split airport connections, but they also have split terminals at LGA with no airside connection that I'm aware of.
Ironically, it may be easier to make a connection at LGA from DL's regional operation to the US Shuttle than it would to connect to the DL shuttle... The original slot-swap plan from 2009 included US moving into the MAT, but that idea seemed to have died along the way. The MAT is definitely more convenient for the shuttle crowd, but kills off any reasonable online connects.
- The US operation had turboprops, which offer lower operating costs, lower landing weights, and arguably, lower complexity. DL's use of RJs will possibly be seen as more favorable by customers, but has higher fuel expenses, higher landing fees
Throw all those together, and I'm not convinced that it will work out quite as nice as EWR does. It's better than nothing, but at a cost.
the simple fact is that data shows that LGA and JFk are the preferred airports for NYC originating traffic; in market after market - I believe including IAH to NYC - LGA where permissible and JFK if LGA is not allowed by the perimeter rule - is a larger market than from EWR (and not that EWR-IAH is a UA/CO hub to hub market).
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CO built a great operation at EWR and it is a full service hub for NYC - but DL obviously doesn't believe it needs to have a one size fits all in a market that is more than twice the size of just about any other market in the US. DL simply needs to serve the top markets from either LGA or JFK - and in some cases by serving many of those markets from both JFK and LGA, DL has a higher share of the NYC local market than CO/UA can have from EWR plus to/from its other hubs from LGA/JFK.
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But let's also keep in mind that DL's obvious strategic target is far less attempting to pass UA in NYC but rather to remain on par... and DL will move a signficant amount of revenue from other carriers who have previously been stronger at LGA and/or JFK.
Let's also keep in mind that those who say that US failure at LGA bodes poorly for DL fail to recognize that US never served anywhere close to the number of business markets that DL does - and AA has at times - and both AA and US pulled down a number of high profile if potentially low yielding markets such as to/from Florida.
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RJs are still lower CASM aircraft than the turboprops that US once flew and large RJs flown by regional partners do have CASMs comparable to some mainline aircraft... they can compete quite effectively.
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I'm sure a large part of DL's decision to move forward with the LGA hub is the success DL has had in LGA-ORD, attaining a 15% market share and receiving average fares up against AA and UA which have large hubs and huge historic presences in the market - even though DL basically has served the market on a point to point basis using E170 class jets.
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DL never has been interested in creating connections to/from the BOS-LGA-DCA Shuttle and, again, their success with LGA-ORD combined w/ AA's decision to remove AE service probably has allowed them to reconsider that it isn't worth diluting its revenue in the Shuttle markets by adding capacity... and again, some of those markets can be served on a connecting basis over JFK where DL will retain sufficient capacity to serve those connecting markets it wants to serve and where it competetively makes sense to do so.
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Of course any new business venture involves risk - just like life. But DL has shown that it has a pretty good sense of the correct balance between risk and innovation.
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BTW, DL has launched a fare sale for the new markets... presumably the schedules are now in all distribution channels.