At the end of Infinity Gauntlet, Thanos was no longer a nihilist villain obsessed with getting mystical macguffins in order to convince Mistress Death to fall in love with him as he learned that he is his own worst enemy and Death will never love him, and he became a reclusive farmer and Explorer who would occasionally help Adam Warlock and other heroes save the universe from a common threat. Thanos got to grow and change under Starlin.
Problem is. Bendis and Hickman regressed Thanos into a thuggish space warlord similar to DC's Mongul, as to synergize with the character's appearance in the MCU. And even before then, Starlin didn't like how Thanos was depicted in the Ka-Zar and Thor comics and he made Infinity Abyss to explain that the Thanos from Ka-Zar and Thor was actually a pair of rogue clones.
The two different paths are so at odds with each other that Starlin's recent stuff has been pushed into alternate realities by editorial.
Thanos is too popular and too important to Marvel to just let Starlin write him when he feels like it. Plus, the redeemed Thanos can't be the Darkseid-style villain Marvel wants the character to be.
But there are also the die hard Starlin fans, who hate seeing Thanos depicted as a brutish villain, so Marvel throws them a bone with the OGNs.
It's kinda like what Renew Your Vows was for the Spiderman fans who hated One More Day or what Fallout New Vegas was for the oldschool Fallout fans who hated Fallout 3, how it is appealing to different groups.
For the record, I don't have much issue with Starlin treating the Avengers like Yamcha since Infinity Gauntlet is NOT an Avengers story.