I’m sure DL could open another SC in JFK T4. Whether it’s 25,000 sq ft, 10,000 sq ft, or 75,000 sq ft is irrelevant if the size of the space can not accommodate your normal passenger volume and flow.
As for ATL B, I have started to avoid that club after my first few novelty visits. My cell phone doesn’t get signal in there (AT&T). Does anyone else have that problem? I’ve always looked for threads on that issue but never see any and can’t believe I’m the only one with that problem. Luckily in ATL it’s easy to just move on to another SC. If I’m departing from B, I’ll go to the A SC, and if departing from any other concourse, I’ll go to the one for that particular concourse.