They will be good. It doesn't take much to beat up on the Big 12. They'll never contend for anything meaningful outside of a Big 12 championship. But realistically for Colorado, that's pretty good. He will likely get a better job somewhere in a few years, and they will revert to the mean.
He won't be able to "out talent" everyone if he ever steps up into the SEC or Big 10. Now that will be an interesting thing to see. I don't believe the coaching chops are there. His coordinators are decent, but nothing special imo. When you have talent a lot of things can be hidden.