The Difficulty Has Not Increased In Casual Content.

[Ameela]

And how long would that take them when the expansion drops? In the meantime, they're falling behind in leveling their gatherers and crafters into the next tier. It would be an incredibly bad business decision on SE's part for their solution to be "Sorry, you'll just have to play catch up at the beginning of each expansion because the regular players think the MSQ isn't challenging enough for them while you struggle to get through it."

You have 6 months between the release of the last batch of story quests and the expansion, that should be more than enough time to catch up considering the MSQ in each patch takes maybe 2 hours at most.