I don't think they will "let you start post-EW" as in like... you won't be able to queue for EW raids or get those in roulettes until you have done ARR, HW, SB, SHB, and EW. I think MSQ 2.0 would be some new thing, as in, when 7.0 hits:
MSQ 1 stays as is. To do UCOB you still have to beat neo exdeath stormblood savage tier, MSQ 1 dungeons are all the stuff we have now, etc etc. You'd still be able to experience the hydalyn & zodiark ark.
MSQ 2 is some new story. The raid tiers in 7.x will be at the end of this new story.
A new player can do MSQ 2 and get to the endgame raids of that expansion without doing all the old MSQ 1 stuff (but it is a bunch of content they can go through in their spare time if they so desire / want to eventually do old ultimates, etc).