It requires a lot of work to:
-unflag everything (sidequests unlock as you progress through MSQ so all these will have to be updated) and add new flags based on level this will also include retainers/gc joining and thus mount unlock
-add new side quests including all translations to any content that was mandatory for MSQ we are talking 15 new dungeon(this is just the leveling ones which means context to the Aery/Vault/Doma Castle are really lost), 3 ARR primals (Mog/Levi/Ramuh), 2 HW primals (Bis/Rav), 2 SB primals (Laksmi/Susano),
-outright change encounters/dungeons/questlines why because Shiva/Nidhogg/Thordan/Shinyru/Omega/Tsukiyomi/Eureka have heavy story significance and well you either get locked out of these so thats 17 fights you cannot do without dong a massive sidequest with all existing cutscenes, fedexing that is still there which also means your locked out from all ponies/birds/dogs and their respective completion questlines their weapons etc etc (and if you think most people would go through MSQ to repeat old content they have done just to do these fights well just XD) or you strip any speech and VA in the fights or you risk spoilering story
-Flying/Diving/inn rooms/levequests/guildhests unlock changed
-Zone access changed you cannot enter the HW and SB zones without MSQ progression after all
-Beast tribes changed specifically HW/SB as they require multiple sidequestlines but also use MSQ with references especially Ananta can't exactly redecorate a bridge if Garleans still holding it XD