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The revenue from credit card programs for legacy US airlines like American Airlines, Delta Air Lines, and United Airlines is significant compared to other sources of income. Here's a breakdown based on available information:

Revenue from Credit Cards: Delta Air Lines, for instance, has generated substantial income from its American Express credit card partnership. In 2023, Delta was expected to collect nearly $7 billion from American Express, which was nearly equal to the revenue from flying passengers in 2022. This indicates that credit card revenue can be as significant as, if not more than, their core passenger transport business. Similarly, in 2019, Delta reported $4 billion from selling miles to banks, which was 14% of its operational revenue. United Airlines also benefits significantly from its MileagePlus program, with a reported $5.3 billion from selling miles in 2019. American Airlines had $5.6 billion in loyalty and related revenues in 2019, including credit card partnerships.
Impact of Banning Airline Credit Cards: If airline credit cards were banned tomorrow, the financial impact on these airlines would be considerable:

Profitability: The airlines would likely see a sharp decline in profits. For instance, Delta's credit card deal with American Express was projected to hit $7 billion annually by 2023, which is a significant portion of their overall revenue. United and American also rely heavily on their loyalty programs for revenue, with loyalty programs accounting for a large part of their market value and operational profits. The loss of this income would directly affect their bottom line.
Loyalty Programs: Without credit card partnerships, airlines would lose a major source of customer engagement and loyalty program funding. This could lead to a decrease in customer retention and an increase in competition for direct ticket sales, possibly affecting pricing and profitability further.
Alternative Revenue: Airlines might have to find alternative sources of revenue or restructure their loyalty programs to compensate for the loss. However, this would not be immediate, and the transition could lead to a temporary or even permanent dip in profitability.
Market Response: Given the high margins on loyalty rewards compared to the core flying business, a ban on credit cards could lead to market adjustments, such as increased fees for services that were previously perks of the credit card programs, or a shift in how loyalty points are earned and redeemed.


In summary, while these airlines still generate significant revenue from passenger services, banning airline credit cards would likely result in a noticeable reduction in profitability unless they could swiftly adapt with new revenue strategies or significantly increase other income sources. The exact impact would vary by airline, but for all three, it would be a substantial financial hit.