What low-cost carriers in the US fly Airbus exclusively? There's Spirit, which is notoriously horrible, and I saw someone mention JetBlue elsewhere in this thread. Any others?
Spirit also gets an unnecessarily bad rap. Yes, their seats are tight, but they fly very few true long-haul flights, so it's not that big of a deal, and the Big Front Seat is probably the best value "premium economy" experience in the US. Prices for food and drinks in flight are more reasonable than they would be in the airport. In my experience, the only really unpleasant things about flying Spirit are:
1. Long lines to check bags (solution: don't check a bag, just pay the $5 extra to carry it on)
2. Other passengers that disregard plane etiquette (listening to music without headphones, etc.) or just complain a lot.
https://www.cbsnews.com/news/allegiant-air-the-budget-airlin...
https://en.wikipedia.org/wiki/Allegiant_Air#Fleet
I think the company's startup plan was to use cheap/old fuel-guzzling aircraft to get the business off the ground: it's cheaper to pay for fuel than interest payments on new jets. Now that they've built a stable-enough business they've transitioned to newer planes. Wikipedia says the company's first brand-new jet was purchased in 2017, which was 20 years after the company was founded.
Many of the issues Allegiant was cited for were also not only due to the plane being old, but also because Allegiant's maintenance crews (which apparently are very understaffed and underequipped) totally missed the issue and erroneously cleared the plane to fly.
Allegiant's issues may have been partly due to old planes, but as evidenced by Delta/American, if Allegiant had a decent maintenance operation the old planes would not have been a problem. The fact that they still had issues does not speak well of their overall safety program, which is more than just the types of planes they fly, and also affects their current fleet.