bullcrap.
DL didn't put 717s on ATL-EWR because it was a good place to train crews.
they put 717s into EWR because the plane fit the market profile at the time.
DL pulled back on Japan-beach market capacity because they recognize that in order to push yields up, you reduce capacity.
I'm sure you also wouldn't get it but DL was one of the few airlines that flies Japan to the beach markets that didn't see its yields decline in the last quarter. And DL's yields between the US and Japan are also not falling while other carriers' still are.
it is always a risk when a market leader chooses to cut capacity. AA did it on JFK-LAX and DL added capacity.
JL has actually added beach market capacity - and their yields went down but their total revenue went up. They are comfortable that they can increase their revenue via beach markets.
JL is watching its value as a connecting airline at NRT fall so they have to grow revenue someplace. DL has a lot of options as to where to deploy aircraft. DL thinks it can do better with its aircraft than to throw that capacity back into Japan-Hawaii.
whether you understand it or not - and you clearly don't - DL is making profit maximizing decisions that allow it pull capacity from the market, push up yields, and still maintain market dominance.