The Kanter deal absolutely wasn't a mistake and the fact that they got through the Spurs series should put that train of thought to bed. They had to re-sign Kanter. If they didn't re-sign him, they wouldn't have been able to replace him with a free agent because they would've been capped out. They would've lost an asset for no reason at all.
The Thunder didn't make a "mistake" in retaining Kanter; this was just an unavoidable, unfortunate situation. I'd rather see them trade Kanter, too, but I always knew, as the season progressed and especially during the playoffs, that they were going to have to trade one of Ibaka or Kanter, as they're the two most expendable big money pieces.