The problem is fit. Kevin Durant is a 3 and Jeff Green's best position is at the 3. If they had just moved Ibaka to the starting 4 a while ago, they'd have been all set with Krstic at the 5 with new addition Nazr.
I don't think so (and obviously, neither did Scott Brooks or Sam Presti).
The lineup of Perk / Ibaka / Durant / Thabo / Westbrook gave up 99.62 points per 100 possessions.
The lineup of Krstic / Ibaka / Durant / Thabo / Westrbook gave up 114.11 points per 100 possessions.
That difference is defense is vast. For perspective, the 99.62 defensive rating is better than Chicago's (which was first in the league). The 114.11 would have been worst in the NBA, 1.5 points per 100 possession worse than Toronto's league worst.
The Thunder have also been one of the best defensive teams in the playoffs, leading all playoff teams in FG% against. To chalk that up to coincidence, or to "they would have been fine with Krstic next to Ibaka", just isn't in tune with the reality of the situation.