[teslabox]

Allegiant started out with old DC-9's/MD-80's, and a few 757's. They were all replaced with Airbus A320's by November 2018. Most of Allegiant's previous safety problems stemmed from using old jets.

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.

[txcwpalpha]

Delta is also known for using old jets (including MD-80s, a lot of 757s, and until 2014 the DC-9. 25% of Delta's current fleet is these old planes) and is famous in the aviation world for having a stellar maintenance program and few safety issues with them. Up until ~2014, American also had several hundred MD-80s (and still has many 757s) in its fleet, and yet it still didn't have nearly the rate of safety issues that Allegiant has.

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.