The trend is not unique to Delta. Airlines across the industry have adopted advanced technology and revenue management systems to better predict what customers will pay for certain seats. As a result, paid upgrade offers have become common after booking, reducing the number of complimentary upgrades available.

"In terms of our [upgrade] standby list, yes, there's a long standby list, and we have a lot of premium customers," Hauenstein acknowledged.

A similar pattern has emerged with Comfort Plus seating—Delta's extra-legroom coach section—which is also in high demand among Medallion elites. Depending on their status level, these members can select Comfort Plus seats at various intervals before departure; however, these seats often fill up quickly due to growing membership in the SkyMiles loyalty program.

"Most of our elite customers are allowed to upgrade into those products at the time of booking, and we didn't have enough of those," Hauenstein told analysts. "If you look across the spectrum, we were generally sold out of Comfort Plus early in the booking [window]."

To address this issue, Delta has expanded its Comfort Plus offerings by adding more such seats on new aircraft deliveries and retrofitting older planes. The airline reports a 25% to 30% increase in overall premium seating as a result. A spokesperson confirmed that retrofits are complete on several narrowbody models—including Boeing 737-900ERs, Boeing 757-200s (75D and 75H), Boeing 717s, Airbus A319s, Airbus A321neos, and Airbus A220-100s—with additional work expected on other models by next year.

Hauenstein emphasized the importance of maintaining perks for Medallion members: "Now being able to increase that so we can accommodate more of our most premium customers with premium offerings at time of booking," he said, "I think we see that there are many, many more opportunities in premium in the coming years."

Delta is also updating cabins across its long-haul fleet by expanding its Delta One suites and plans to introduce an Airbus A321neo featuring 44 first-class recliners soon.