Also I'm not aware of airlines flying only and solely the A320 Allegiant and EasyJet are, IndiGo was for years A320-only, and Batik Air (part of the Lion Air

Also I'm not aware of airlines flying only and solely the A320 Allegiant and EasyJet are, IndiGo was for years A320-only, and Batik Air (part of the Lion Air group) will be soon. Edelweiss has A330 and A340 as well but it's relatively easy to move between the 320/330/340.