Yeah, besides the eye test, metrics indicate that Bertans is the much better defender. And don't come here trying to say "well, those numbers are playing the 4" because A) he played plenty of 3 and B) I would argue that playing exclusively the 3, his metrics would be even better.
And just think my friend: neither is a great one on one defender, but if you had to choose, who would you rather have defending the Kawhis, Durants, Lebrons, Klay Thompsons of this World. Forbes, Belinelli or Bertans? We all know all of them would get killed, but, heart to heart, who would get killed the least? The answer is Bertans. You know this, I know this, everybody knows this. There's no reason to keep shielding for Pop on this one. The three are out of the roster, it is done.
So, considering than Bertans would get killed the least on one-on-one defense, paired with the fact that he would also provide above average help defense, and culminating with the undeniable truth that he was by far the best offensive player of the three, there's just not a single reason to have had Forbes and Belinelli getting minutes over him all these years. None.