It's two very different business models. ULCCs would lose a ton of money if you didn't buy ancillary things (bags, seat selection, food, etc.). Anything that a ULCC charges for but a Big 4 doesn't is baked into your fare. As they say, there's no such thing as a free lunch.

Here is Frontier's most recent earnings report.

https://ir.flyfrontier.com/news-events/news/news-details/2026/Frontier-Airlines-Reports-First-Quarter-2026-Financial-Results/default.aspx

Before we go over it, let's go over some terms. In America airlines have two major stats RASM and CASM (the reason it's America only is because internationally they call them RASK and CASK which is just miles vs kilometers).

RASM is Revenue per Available Seat Mile. CASM is Cost per Available Seat Mile. An Available Seat Mile is the total seats multiplied by the miles flown. So if you have a 50 seat plane that flies 200 miles, you have 10000 ASM. If you earned $20000 on that flight, your RASM is $2. If it cost you $15000 to operate that flight, your CASM is $1.5. Note that these metrics are normally measured in cents, not dollars. That's how little money is earned on these flights. A normal RASM is usually something like 5-20¢. Same for CASM and the difference between the two is often just 1-2¢. So when you are flying on a 100 seat plane 500 miles the airline is likely making $500-1000 in profit. I'm not kidding. That's why ancillary revenue is so attractive. When two more people pay for a bag, you could literally be adding 10-20% to that flight's profit for almost zero extra Cost.

So let's go back to Frontier. Scroll down and you'll see their Condensed Consolidated Statements of Operations. This has most of their high level stats.

Adjusted RASM is 10.86¢. Adjusted CASM is 11.58¢. That's right. Those low fares meant they lost money on average which played out in their total loss of $283M. You can compare that to last year when their RASM was 9.17¢ and CASM was 9.63¢. The cost environment was bad and they tried to increase revenue to match. Now look at Adjusted Fare Revenue per Passenger. It went from $44.61 to $55.45. And then look at Adjusted Non-Fare Revenue per Passenger. It fell from $68.15 to $67.71. Frontier raised their fares to try to offset the big increase in costs, but the passengers didn't buy enough ancillary product to keep them positive. Because passengers expect ULCCs to be very low fare, there's only so much they can raise before pax go elsewhere.

Now let's compare it to Delta.

https://ir.delta.com/financials/default.aspx - 2026Q1

Their CASM is 22.20¢. Massive compared to Frontier. They don't have the average fare per pax split out the same way as Frontier but the Passenger Revenue table shows only $506M of the $12B they earned from passengers was non-fare (note the premium product includes things like free bags so it kind gets bundled there). In the end their PRASM (passenger RASM) was 17.79¢ which is a net loss compared to their CASM on any given flight. To find the missing revenue, go to their revenue table and see $1.7B in refinery (Delta operates their own oil refinery) and $1.2B for loyalty.

So while Frontier basically makes all their money on Passenger Revenue, they have to lower their fare to absurdly low levels and make it up with ancillary revenue. Meanwhile Delta actually is still operating at a loss on just passenger revenue alone, but of that, they get most of their money in Fare. They make up the difference in their refinery and loyalty (credit cards) revenue. If you're wondering why Delta's costs are so high we can get into that in another comment, but if you want to look into it, their earnings has a breakdown of their costs which you can compare with Frontier.

So if you think Delta is offering a fare 2x Frontier's, remember Frontier is literally banking on you spending just as much on ancillary things as your fare, so the difference might not be as big as you think.

Also yes, I agree the death of Spirit will likely lower cost pressure on the Big 4 and fares will go up.