DL's great, but they still have the oldest fleet and actively buy airplanes at the end of production runs (738/9ERs) or that couldn't sell previously (A220s when Bombardier), or keep them forever, like the current MD-88s and MD-90s. But DL does make an effort to address the customer impact I don't see elsewhere. The ancient A319/A320s look brand new with the refurbs, which were done thoughtfully and make 20 year old planes look like they just rolled off the line.