Raiding is becoming less and less optional

[Iscah]

I think the recent digest for the Last Live Letter actually answers this to a degree. For new Players going through the story, once they finish the quests in 2.55, they will get a notification to complete the Crystal Tower story if they have not already. It was not specified if players would be blocked from entering Heavensward, or that block would be in place for only 5.3 quests, but with the way it is worded I believe it is in fact the latter.

If that's the case then I suspect the updated 2.X quests might highlight the start of the quest line, a bit like more recently they've made a point of things like the Four Lords and Sorrow of Werlyt quests within MSQ. It could be as simple as "that man over there is giving you odd looks; maybe he wants to talk to you."