From my perspective this game was built more for it's beauty and feature (more for the free roam side of things) the missions in V unlike the previous titles are more of a add-on to this game.
Of course that's my view on it so it's easy to disagree but like I said the missions are more of a side feature in GTA V so for me it doesn't seem "rushed"
I don't think that the missions are only some sort of side-feature in V. This is Grand Theft Auto after all, a series that is popular for its amazing singe-player experience. Missions are part of the story, which in turn is part of the single-player experience. The story, the missions and especially the freedom are what makes GTA the popular series it is ever since GTA III. By the way, the OP was only talking about the last mission, and not all of them.
Anyway, to come back to your point, I doubt that the missions got less focus than in any of the previous titles. Let's look at it from this view; San Andreas is the GTA with the most features, whether they are all useful or not is to each his own, and missions still played an important role. In fact, many of them even incorporate plenty of those features. And I am sure the same is the case with V. If the game feels rushed to you when it comes to the missions, then that's probably because Rockstar tried too hard to incorporate in-game features in them. It's an issue we already know from San Andreas, and it wouldn't surprise me if the same were the case with GTAV. I don't know though, as I haven't played the game yet.