Exactly this. It's amazing how suddenly solid the Spurs' defense is once Jakob doesn't have to constantly get out of position to guard the opposing PFs, isn't it? And yesterday, Aaron Gordon ate us alive in the minutes KBD wasn't on the court matching him, whilst Jokic on the other hand (who everyone was predicting would "light Jakob up"...) had a very pedestrian game by his standards.
It's plain as day to see Keldon doesn't cut it as a PF, on either end, but especially so on defense. He doesn't only lack the size to defend the position; he lacks the strength, the positioning ability, the rebounding and boxing out instincts, the help defense rotating instincts. He's simply not a PF, or at least not a starting one, and it hurts the Spurs every game he pretends to be one. A difficult conundrum for the Spurs to be sure, given how relatively good he is on offense, and being a fan favorite, and a Gold medalist, but he's just not a PF, and the Spurs can't depend on KBD every game neither.