Well, it's not as simple as that.
First, there's honestly no indication that GS would win anything more with Durant than Boston would with Durant. They have many more questions about fit, chemistry, and the surrounding cast than Boston. Sure, talent is nice, but there are several recent teams with high levels of talent that lack some combination of fit, chemistry, or a strong supporting cast that haven't done crap.
Also, the question itself is flawed. It should be something like this:
Should Durant choose to still lead a team as the best player and still possibly win multiple MVPs and title versus give up MVP hopes for a slightly better chance at a title by joining a team that has:
A) a two-time reigning MVP
B) three All-Stars
C) just beat the all-time regular season record
D) established itself as a legitimate title contender without Durant.
That makes the question a bit more realistic of what's going on.
Also, I asked this question earlier and nobody answered it - has there ever been a team where the previous MVP has teamed up with the multiple-time reigning MVP to take down the team of another former multiple-time MVP that neither of them seem to be able to beat?