If they really believed they could win 3 of the next 4 with Durant I bet they would pull the trigger. Sustained winning is good, but the Warriors would be the GOAT team with that kind of success (in addition to what they've already accomplished).

I suspect the reason they aren't is because their title window could be short in such a scenario, although they'd be big favorites right now. Klay and Dray are unlikely to be enough as your #3 and #4 in a couple years, and without Wiggins and Poole to step up as they age, you'd basically be putting all your eggs in Steph and KD beasting the league with role players and staying healthy. It's possible, but it's also a brittle strategy.