Simmons is already a more productive player than Anderson. Anderson used to be the better NBA player. That's how bad Simmons playing was when he first came in. He had all the athleticism and hardly any of the skill of an NBA players. People have also been saying simmons has 'fullfilled' his potential since his first year, which is just a re-skin of the original argument that he doesnt' have the potential of a younger player.
He's a legitimate backup in the NBA, whereas Anderson has not played like one. Metrics, I assume you're talking about hiding in the corner and picking up rebounds that fall between him and another spurs player. He has a good plus minus, because he plays with arguably the best starting front court in the NBA. kawhi and LMA haven't had too many games where they are outscored by opposing starters, thus the great record. When he plays with Manu and Patty, manu and patty have been lights out recently and he has just been a bystander. That's a different story with Simmons in.
When anderson is on the floor, the other team doesnt notice, when Simmons is, the other team realizes and HAS to be ready for him. Simmons has the ability to make the other team notice he's on the court, and mentally have to adjust for it. Anderson is nothing like that at this point.
I've been a fan of anderson since we drafted him, as he offered unique characteristics, but as I've said before, his character does not seem like one who wants to make the most of his opportunities. He seems much too soft, which is why, in my opinion, he excels in the d-league where most the players are as slow as him or can't outjump his reach and its' like he's playing with little brothers. He has had time to show he wants to be an integral part of the system, and has had more opportunities than most young guys get coming into the spurs system. He hasn't made much of them to this point. I still think he could develop, but to me it's more of a mental thing and he can't afford to be weak there as he's already physically outmatched by players.