Spirit is the airline that everyone loves to hate. And paradoxically, it’s the future of air travel.

In its latest airline capacity report, FlightGlobal found that of all U.S. carriers, Spirit’s growth for July was by far the most robust, with an increase in the number of seats for sale of 18.6 percent year-over-year, and an increase in ASKs (available seat kilometers) of 23.9 percent.

Granted, those increases are from a relatively small base. But if the carrier were to continue expanding at that rate, it wouldn’t be long before it overtook Alaska and JetBlue, two airlines with decidedly different business models. (For context, American added no new seats in July, Delta increased capacity by 1 percent, and United was up 5.8 percent.)

It’s not just Spirit. Frontier Airlines, Spirit’s ultra-low-cost cousin, is growing at a torrid pace as well. This week, Frontier announced service to 21 new cities, expanding its city coverage by almost 30 percent and doubling the number of served routes. According to the airline, “By next spring, Frontier will offer low fares to 90 percent of the U.S. population, which is expected to bring more than $1 billion dollars in savings to consumers through lower airfares.”

Spirit and Frontier’s expansion isn’t the only sign that travelers have embraced the ultra-low-cost carrier value proposition: extremely low fares in exchange for barely tolerable legroom and a long list of add-on fees. So popular have the unbundled fares become that American, Delta, and United now offer their own stripped-down fares, in a new airfare category called Basic Economy.

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Here’s how United described its Basic Economy product:

To further meet customers’ needs and provide more options to price-sensitive travelers, the company announced the introduction of Basic Economy fares. This new offering provides customers the option of paying the lowest fares to their destinations, while still receiving the same standard economy experience, including food, beverage, Wi-Fi and personal device entertainment, with a few key differences.

Among those “key differences”:

• No advance seat assignments
• Carry-on bags limited to one “personal item” (waived for elites)
• No ticket changes
• No elite-qualifying miles, segments, or dollars
• No upgrades

While the Big 3 currently offer a smidge more coach legroom than Spirit or Frontier, the average seat pitch among what used to be called the full-service airlines continues to decrease, to the point where the difference has shrunk to as little as a single inch.

For the average traveler, what it amounts to is a race to the bottom. Lower prices, but less comfort and less convenience. It’s what we as consumers have voted for with our pocketbooks.

Reader Reality Check

Have the airlines struck the right balance of price and service?

After 20 years working in the travel industry, and almost that long writing about it, Tim Winship knows a thing or two about travel. Follow him on Twitter @twinship.

This article first appeared on SmarterTravel.com, where Tim is Editor-at-Large.