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.