Tim is playing against the best big the opposing teams have, every night. If Boris had to start against those guys, you'd have a ing coronary and die.
Tim STILL averages over 11 REB per 36 minutes. The very BEST Diaw has done for a season is about 7 REB per 36 minutes, and as a rule he averages about 5.5 boards per 36. Put Diaw into Tim's role, against the opponent's starting bigs, and the REB difference alone would have the Spurs in a hole to start every game.
Duncan STILL gets over 5 FTA per 36 minutes. Diaw gets about 1.7. Tim is playing a big man's game in the middle, and forcing defenders to foul him. Diaw is playing a totally different game, against much less skillful big men (on average). He's not grinding down beasts in the paint.
There's no subs ute for a legitimate big man, especially at the start of games. Later in games, when the lineups are mixed on both sides, it's a different story. But in the early going, Tim is holding down the middle, and he's still one of the best in the league at it. Boris is an incredibly skilled player, and I love watching him do his thing. But he's not going to start for Duncan, unless Duncan is injured. Period.