There is a fairly long running thread on the DL forum about how much some people expected WN to make a big splash in ATL that hasn't quite materialized at this point. Right now, WN is more worried about watching its multi-billion dollar investment in FL go down the toilet as a result of labor woes (like this has never been a problem for other airilnes in this industry...). WN also has its hands full trying to kill off FL in DEN where WN's average fares are well below what they are in more stable markets such as SLC and ORD to the same destinations, even on a mileage adjusted basis.
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The simple fact is that DL has decided what markets it intends to defend against anyone and they have been able to do that quite successfully in the 30+ years of deregulation.
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DL closed DFW and used those resources to build up NYC; current cuts to MEM and CVG will provide even more resources for DL to grow NYC, a market in which DL is very close to growing to have a larger local revenue base than ATL.
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For whatever reason, DL decided they wanted to add PHL-LAX, US thought they would add CLT-SLC, DL has announced the same thing with a schedule more conducive to business passengers - and the competitive juices that used to flow freely between DL and US are flowing quite freely again.
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We'll see how it all works out.
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DL has a pretty good track record at defending its turf and in winning competitive battles with US; you need only look at the number of east coast markets where US was once much stronger and see where DL and US are in relation to each other now.
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The simple fact is that DL is a highly competitive airline and has grown immensely because of its willingness and ability to go after other carriers' core markets. That reality is well known in the boardrooms and executive offices of airlines throughout the industry, including in Tempe, which is probably why they tried to buy out DL not that long ago.
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Whether I or anyone else points out that reality doesn't change the fact that it is what it is.