Looking at the last ten years of Marvel movies, we’ve been through a lot of villains. A lot of them didn’t last more than one fight with the main character.
It’s not that I don’t trust the Russo Brothers (or that I don’t want to trust them). And it’s not that I don’t think that Thanos doesn’t have potential to be a GREAT villain in Avengers: Infinity War. But it is very hard to not buy into the online fandom rhetoric that Thanos hasn’t really done anything to show that he means business. He is willing to work with dangerous people to get what he wants. He technically ruined the lives of both Gamora and Nebula. And I don’t underestimate the amount of damage Thanos could do with a loaded Infinity Gauntlet. But most of what we know about Thanos in the MCU is by reputation.