Durant made the Warriors better at least in 2017 which is one of the best seasons of all time going 16-1 in the playoffs, and the Cavs team they beat easily was really good, arguably better than 2016. 2018 and 2019 is more debatable, he was hurting their ball movement to some extent. Remember the Warriors finished off the Rockets in 2019 and then swept the Blazers without Durant.
With that said he's supposed to be a top 20 player of all time, and he was a perfect fit for Golden State's lineup. Any other player on that level added to 2016 Warriors should have made a mockery of the league. For example if you added a player like prime David Robinson as their center I personally feel the Warriors win like 75 games or something in 2017 and are cruising much more in 2018 and 2019, probably allowing them to get less injured.
I believe Durant is one of the greatest pick up basketball players of anyone in NBA history, he has been roasting teammates his whole career. However that one on one skillset doesn't translate quite as perfectly to NBA, although still very good.