I think ultimately the problem is that Starlin wanted to evolve Thanos beyond being a villain... but from marvels perspective, he was one of the best villains in the MU and one of the few with legit credibility. They don't need another hero... they have plenty. But they really need good event level villains that readers buy as super credible threats. And apart from maybe Doom, Thanos is probably the biggest and baddest they have.
Had Starlin kept him a villain, there wouldn't be this inevitable back and forth.