If you are speaking only about ATL, then NK is not the threat that it is in DTW or MSP, but NK absolutely does matter to DL in every market. The reason why DL is more susceptible to NK in DTW and MSP is because there is no LCC. but given the size of WN's cuts in ATL and the increase in their fares, NK sees an opportunity for the low value passenger.
remember that NK almost always starts a market with just one flight a day or less which is what they are doing in several new ATL markets including BOS and MCO and others. but when they fill that flight, they add another. that is what has happened in DFW - and in some DTW and MSP markets. and when you have more than one flight in a market, you do grow much faster in terms of ability to pull passengers.
Basic Economy has nothing to do with DL's costs... DL is very good at matching capacity to demand and not carrying unnecessary costs. Basic economy is about adding a fare point to allow DL to compete with the ULCCs.
You can pull a fare extract in every market and tell me where DL does or doesn't have fares but I suspect you will miss them.... and I can also assure you that DL is competitive. Given that we are now in peak spring break period, DL is offering very few deep discounted fares in any market... they don't need to. There will always be a certain amount of very low fare passengers that ULCCs can pick up because they move their planes in and out of markets... but DL will be competitive with carriers that are in DL markets on a sustained basis.
and no I didn't say that DL is flooding any market competitive with NK.
the most significant location that DL is adding capacity is in SEA and west coast markets where DL has clearly decided that a partnership with AS is not going to work and so DL must have a presence of its own. that has been discussed a million times.
btw, the Port of Seattle is doing more than its share of pushing for new and expanded facilities at SEA.