The initial mistake was EA's in giving them such a limited development window. They treated ME3 like it was another generic sci-fi action game when it was an expansive RPG just like its predecessors. Games with that much content to them take a lot more time than typical shooter games do. They made the same mistake with Dragon Age II; fortunately it seems they've wizened up a bit by giving Dragon Age: Inquisition a lot more room to breathe.
This is my take on it too. EA isn't to blame for ME3's craptastic ending, nor any of the other dumb creative decisions in the games, but they are to blame for putting Mass Effect on a 2-year development cycle. That might be enough time to churn out a shooter with an 8-hour campaign that nobody actually plays, but for games with the scope and detail of Mass Effect that isn't enough time.
I'm hopeful for the next ME game because it seems like they've been given extra time to work it out, which seems to have resulted in the return of exploration and vehicles (if these rumours are true). Besides, this new game will be a fresh start in terms of story and wont have to write its way out of a corner in the same way ME3 did.