There's nothing wrong with that, Ibaka is paid well, so he needs to have good minutes and he is already a veteran therefore a coach would choose him than any other player with the same position. Zubac is good but he needs to improve, look at the line up of the Clippers, they all look veterans and that's the reason why they are winning.
I don't think that size of Ibaka contract have to do anything with the amount of time he is playing. After all, Ibaka has only 2y/19 million contract so its not like he is on supermax. Ibaka being starter has to do with the the fact that he is pretty solid at 3 pts shooting so he is able to stretch the floor and give more room to Kawhi and Paul George. At least that's how I understand this decision but as a fan of Zubac I don't have to like it
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If you'll look into his statistics here
https://www.basketball-reference.com/players/z/zubaciv01.htmlYou'll see that there's not much improvement, his playing time is still almost the same as the previous season, I think it's okay for him to take that rule, he is still young, he doesn't need to rush things and I can say he is not yet ready to be a starter, no offense but my honest observation only.
I am perfectly aware of the stats. But bare in mind that last season he was starter and had good chemistry with Kawhi while now he is part of the 2nd unit, which doesn't play good at all at the moment. For example, just look at Lou Williams numbers this season and compare it to previous one (he was good though in the last game vs Bulls), so no wonder that Zubac numbers ain't great either.