Thanos and the Black Order wrecking shit is the first act of a movie, not a whole movie. What would the plot be?
It would be the official intro of him and the Black Order. I haven't seen GoTG 2 yet, but if Nova Corps still has the power stone, they'll be the first to get wiped out. That leads to GoTG and the cosmic side of things taking up most of the first one with the ending being the GoTG ending up on earth.
The second would've been The Avengers (and earth) coming to terms with aliens and crap and trying to strategize on what to do. Meanwhile, Thanos begins his move to earth and shows up during the second half or the end.
The third would've just been the main conflict between The Black Order/The Avengers and then the ultimate fallout with the deaths and shit.
After dozens of movies building up to this, I don't think it should all just be resolved in two movies, especially since we don't have any direction for what the next big event is. Call me greedy, but I think with the number of characters there are and the size of this event, three movies would've been fine.