They literally said this in the stream. (Destiny)

by Kahzgul, Wednesday, December 06, 2017, 17:11 (2623 days ago) @ ProbablyLast

Not sure how anyone would expect it to be different. They wanted the strikes to be part of the story so they made them into missions.

I’m not buying the DLC because the game is still unfun and a 45 minute story won’t change that, but this isn’t a valid complaint.

I played for at least 3 hours last night, pretty much straight through the story, and while my friends told me I got near the end, I didn't finish. 45 minutes might be possible if you're a full fireteam speedrunning as much as possible and ignoring the story (and there is one, and it makes sense), but for the average player, you're probably looking at 4 hours (at least) of gameplay just to finish the story, before the midgame and endgame content opens up.