People are conflating two separate subjects....
1. Would the Warriors have won w/o KD the last 2 years? Ehh....They definitely would've had a lot of equity to do so since they would've re-signed Harrison Barnes. It's not like you are replacing KD w/ their 12th man in this hypothetical. But does Kawhi get hurt in the '17 playoffs? does Kyrie still get traded after the '17 finals? (he likely is a non-factor in '18 because he his injury was a wear and tear injury, so he isn't playing for anyone in the playoffs that year) and probably most importantly..What team is Kevin Durant actually on? Who knows. But GS definitely would've been a top contender. There's just tons of other factors that can't really be accounted for.
2. If Durant were to leave the Warriors in 7 months, what would their prospects look like going forward? This seems a lot dimmer. It would come after a 5 year 500 game run and the cupboard would be left pretty barren. Iguodala/Livingston are getting older, Barnes is in Dallas, Bogut is gone. It's just a totally different team and because of their salary cap situation, they wouldn't really be able to improve it very much if he just opted out and left. You're basically left with Curry/KD/Klay and some aging roleplayers. Perhaps their GM could get creative and swing a trade, but doesn't seem easy to do. Also, if KD left, Klay could be right behind him as he is a FA as well and could see the writing on the wall....
It's way too early for me to think about what KD is gonna do in free agency though, but gun to my head I'd definitely make him a favorite to stay. Doesn't seem to have close to gumption needed to make a ballsy move like that. Especally if it is after a 3peat.