They could compete back then, but I guess they don't have the experience back then. And then KD moving to GSW was the final nail in the coffin for them as WB can't carry the team although he has the best statistical numbers for him and won MVP.
KD did the right thing that he thinks when he's transferred to GSW and asked for it. It's because he wants a ring and he did with these guys. Right now, it's hard to see them back again.
Despite all of the other strong rosters, the game has changed a lot and a team with a lot of superstars doesn't fit anymore when they're all playing together.
KD decided to leave when he was only partnered with WB. Probably, the two no longer had a good relationship as there was no clear superior between them, as both were MVP candidates. So Durant was looking for a team that he could lead. Fortunately, Curry is a true sportsman; he stuck with the game plan and let Durant lead the team despite being the superstar of the team. With that chemistry, they won 2 championships together before Durant left.
I think they still have a good relationships back then, but WB failed betrayed when all of the teams that he wanted to go, he decided to with GSW which is the team that they wanted to beat.
And with that, their relationship started to deteriorate and then Durant portray as the villain here. But I think they have move on with their careers already and there is nothing but respect with each other.