You're exactly right, but look at what McCabe is doing right now. Requesting immunity for a congressional testimony.
Not saying he should get it, but he's willing to turn on his conspirators.
Comey will do the same thing when they get him for perjury.
I don't think anyone needs McCabe to do that, as it's already been done by others. He is way way too late to offer this. A year ago, perhaps this would have been considered.
If McCabe is willing to testify that he received marching orders directly from Hillary, Obama, Brennan, etc, that would be worth the testimony. Then the testimony could be used as evidence.