Bundling a fleet sub type in one hub makes sense, that way you save on MRO. The A vs B discussion is as old as the A320 (happy 30th birthday, BTW). As one MD80 pilot (who switched to A320) put it at the MD80 farewell do some years ago: "An DC9/MD80 is like a girlfriend you know since years: you push the right buttons and treat her with respect and she loves to give you what you want. An A320 is a fancy french lady who keeps telling you what you didn't do right and does things how she thinks they ought to be done".
Changing type rating between A & B is to also rethink things.