Let's look at what the Spurs have on the frontline.
Ratliff can block shots, but is old, fragile and mysteriously doesn't play.
Blair can rebound with ferocity, but lacks size and isn't a polished defender yet.
McDyess is a good positional defender and rebounder, or at least he was last year, but can't really step out and guard people on the perimeter, can't block shots and looks very much his 35 years.
Bonner is just a shooter and hustle guy, who doesn't rebound, defend, block shots or anything else.
Mahinmi can run, rebound, block shot and looks to be competent offensively, but mysteriously has entered the Spurs witness protection program.
That leaves Duncan as the sole anchor of everything the Spurs do offensively and defensively on the frontline. At 33 years of age, the guy is working too hard and the load is just too much. We saw him wear down last year, and it appears to be happening again.
Let's face it, regardless of what you may think about Thomas, and despite the fact that he has fallen out of favor with his current team, the guy has all those tangible defensive skills that the Spurs have so very little of. He still needs to develop his offense, but he'd be an instant upgrade at the PF position.