I think where they went wrong is using Vulcan and the X men world retreads. Retro continuity is never popular, it made the cosmic side seem like an appendage of x men, and Vulcan just was not compelling enough a villain after the likes of Annihilus.
I do agree they should have slowed the pace and done more to foster enduring interest in the compelling figures they introduced. There was so much there! I would have picked up a limited series on Cole, Swad and that XXX chick they introduced in Thanos.