If you read Aaron Fisher's book (Silent No More) you can see this process play out in real time.
As much as I would love to hate everything Corbett says (because he's a snake), there is some truth to this.
Others *could* bring other claims if the first claim (Fisher's) would have failed, but that would make it harder to prove the secondary claims if Sandusky walked on the first one.
For instance, while Mike McQueary was rightly criticized for not helping that boy in the shower in 2001, it was *HIS* testimony at the grand jury that was the tipping point. While he should have said something sooner, when he testified the grand jury saw him as a credible witness and things moved in the right direction.
Without McQueary (and other victims), Fisher could have been painted by Sandusky's defense team as a money-grubber, and the jury would be willing to believe it in order to keep hold of their illusions. It was only overwhelming proof that tipped the scale.
That said, it still seems pretty clear that Corbett delayed the investigation until his race for governor was over. He's still a scumbag.
But he grew old, this knight so bold / And upon his heart a shadow / Fell as he found / No spot on the ground / That looked like El Dorado.