Nothing needs to be changed assuming the current level 50 quests add exp.
For old (current) players this pretty much is what it is, if they didn't like doing the MSQ that's just unfortunate as part of gating content behind story elements requires doing the quests in the main scenario, It's no different than gating content like primals and trials behind the story.
For new players this shouldn't even be a consideration.
Like you said to a player just joining the game without any information the "new" areas from the expansion actually aren't new, they're just additional areas.
If anything a new player coming to the game will actually feel a greater sense of progression because they won't be doing a set of storyline quests at level 50 that offer zero exp, the progression experience of a player making their way towards Ishgard from the beginning of the game would feel seamless as there would be no break in leveling from 1-60. It wouldn't matter if anyone else is leveling at 51/52+ in new areas because they would have a large number of MSQs shoveling them exp to quickly catch up.
