It’s too long, simple as that. New players will easily outlevel the main scenario as they are leveling up. I’ve a friend who I brought to this game and he started months ago and has hit level 70 and is barely halfway through Heavensward. It’s way too long, and I’m not interested in arguing that. This thread is going to assume you agree with this and is purely for discussing possible solutions to this assumed problem, and I’m not interested in arguing if it’s a valid problem.

Some ideas to get things started:
1) Cut out the 100+ quests between ARR and HW down to 10 or so composite quests that remove all of the unnecessary parts.
2) Create composite quests for everything from ARR to Stormblood that greatly increase XP and greatly lower the number of quests.
3) Start Shadowbringers players in a special story arc in which the WoL loses their memories and strength and have a special questline parallel to the 70+ msq in which they recover their levels and memories of the past, until at level 80 they have fully recovered and unlocked all past cutscenes at the inn.

More ideas!