It's 100% revenue in terms of earning and burning on the airline itself. But it's actually closer to 0% revenue than many of the legacy airlines in terms of partner earning!

In other words, while there are, for example, several airlines at several hotel programs where the miles you earn are proportional to the spend you do at the hotel, Southwest gives a flat number of points per stay in every hotel program, so hotel partner earning is 0% revenue with Southwest, but some sizeable percentage of revenue with some legacy airlines!

Southwest gives also a flat number of points per car rental.

So the only partners that Southwest has where earning is revenue based are the dining program and the credit card (and that's only regular earning, not the singup bonus, of course).

Even that's not clear (in terms of redeemable miles). The issue: Southwest can get away with it because it's a single-class airline. With miles programs, the number of miles you need for business class (or domestic first class on two-class planes) is only about 2x the number of miles you need for coach class, and the number of miles you need for "true" first class (ie, on three class planes) is only about 2.5x to 3x the number of miles you need for coach.

Simiarly, most airlines have both "saver" and "last seat" awards. In a given cabin, the "last seat" awards take only 2x (AA/UA) to 3x (DL) the number of miles as the "saver" awards.

However, in cash prices, business/first is typically 5x to even 10x the cost of coach (especially if looking at advance purchase). And advance purchase vs "last seat" in cash prices is also about 5x at least on the legacies. (There's somewhat less of a difference between advance purchase and "last seat" prices on Southwest compared to the legacies, which also helps make it possible for Southwest to get away with being revenue based for burning.)

So redemption couldn't easily maintain these ratios on the legacies if it were converted to being closely tied to revenue while keeping cash prices where they are.

... There is, however, an area where legacies are definitely moving in the "more revenuie" direction, and that's the qualifying / requalifying for elite status. Both DL and UA gave already implemented a requirement for "qualifying dollars" in additon to qualifying miles for reaching elite status, making reaching elite status purely by flying a bunch of ultracheap fares no longer possible at these airlines. I think there's more interested in fishing out "cheapskates" from elite status that they are in converting redeemable (not status) mileage programs to revenue programs.

However, there are other kinds of changes (than moving to revenue) possible, with precedents for them: BA / IB Avios points are redeemed based on the length (in point-to-point distance) of each flight segment, on a per-flight-segment basis. That's quite different than the US airlines which require the same amount of miles for flying all the way across the country as for flying a 45 minute hop to a city that's a four or five hour drive away. It can take way fewer Avios points than (BA/IB's partner) AA miles if your flight is a midcon-or-shorter nonstop flight.

BA switched from miles to Avios points a couple years ago. Why is it not possible that some US legacy airline might switch to something like that, rather than revenue-tied redemption?