One of the most frequently spouted "objections" to giving the Carbuncles back to SMN is, lore.
Just for the record, I don't believe there is anything in lore that disallows SMNs from summoning Carbuncles.
So I would like to propose SE return the Carbuncles (upgraded stats-wise to match the Egis) to SMNs via "joke" quests - that require a moderate amount of effort to complete. (Edit:
Maybe give us a glowing Carbuncle Snuggie if we complete the whole quest chain. :3)
"Joke" quests would be like Omake segments in anime. They aren't technically canon and are just for Fun™ - you know the point of this game.
I believe this will appease the "true believers" with regards to lore.
PS:Seriously, we have Miqo'te PLDs tanking in bikinis, Lalafell WHMs healing in their pyjamas ... etc.