I agree they would have gone back to back if Draymond wasn't an idiot but they didn't and when Bogut got hurt, you could see the impact on their defense as it started to splinter. I mean, they still had 2 games, including one at home, to close out and they couldn't do with Draymond back and actually being the best player on the court for them in the deciding game 7.
Now, if you take stock of what the Warriors were expected to do if they didn't sign KD, it was to re-sign Barnes, try to keep Bogut and then work to replace Ezeli (West) and maybe Speights. Bogut was pretty much done after the injury and proceeded to play 26 games the last 2 years. He probably would have a been a non-factor coming back. He was used up.
Barnes would have come back and you never know how a guy is going to develop in different circumstances, but based on a enormous increase in USG% (~10%) he's averaged career best in most top line numbers for Dallas. But there hasn't been a big jump in actual effectiveness. His PER went up marginally, as expected with the ball more, but it's still in the 15/16 range, which is league average. His WS/48 went down by 30-35% and his VORP reduced to basically replacement level. His shooting didn't improve, you could argue it got worse. In other words, Barnes stayed pretty much the same player, he simply had more opportunities to handle the ball. I could easily make a case with this increased USG, his inadequacies became more pronounced and he's a worse player but perhaps this will make him a better player going forward (don't hold your breath). Also, Barnes had an interesting ability to get worse in the playoffs. His all around game decreased in the playoffs. His contribution during those 3 games the Warriors had to close out the Cavs in 2016 when they needed him most? 5-32 from the field, 3-15 from 3, 9 rebounds, 7 personal fouls, and 2 assists.
In some ways, signing Durant saved the Warriors from themselves and yes the Warriors FO have done a lot of great things but they also have been extremely lucky and been bailed out when they make bad decisions. They offered Barnes an extension early in 2016 to stay with the team. 4 years for 64-68 million were the rumors. If he signs this, no KD to the Warriors but he didn't and bet on himself. In some ways, he won...... Dallas paid him 25 million this year and 8 million is a ton of dough. But he plays for a shitty team, puts up empty stats and hasn't appreciably become a better player.
Assuming that Barnes doesn't magically become an All-Star in 2017 for staying with the Warriors and Golden State isn't hiding the Lazarus pit from Andrew Bogut, the Warriors are not the same team. Everyone sees 73 wins and they think it's easily duplicated. It's not. It was pretty much everyone's career year. Steph will never have a regular season as great. He lead the league in WS, WS/48, VORP, TS%, PER, BPM, OPBM. Fucking everything. It was crazy. He'll never have a regular season as good, it was a great player's career year...... by a ton (about 15% better).
It was Draymond's career year, certainly offensively. It was Shaun Livingston's career year. Bogut's career year. Klay's 2nd best year.... etc, etc. You get it. Everything came together in a crazy way and they were never going to be that good again. But ignorant people (not you) simply see 73 wins and think it'll continue into infinity, b/c they're dumb. Interestingly enough, when the Bulls won 72, it wasn't Michael's best year. One of them? Yea, but not his best. It was one of Scottie's best year but not definitively such. Same with Rodman.
All of that written goes to say, yea the Warriors probably needed KD last year b/c Bogut would have been hurt and Barnes would have probably been the same meek player. Kyrie was still great and the Cavs were better offensively. Certainly they needed him this year. Houston probably beats them in 5 with Barnes and Bogut.