The only way these games will be on NBC is if NBC wins the contract for the playoff. And the contract for the playoff runs another four seasons. So if they do this before then, there is one and only one choice as to where the games will be shown, and that's on ESPN. Why would ESPN give up the rights to the college football playoffs, something that makes them money by the truckload, two years early? The only way that happens is if the NCAA buys them out, which might not even be possible based on their contract. And why would the NCAA want to buy them out, rather than just get ESPN to pay them more money?
Maybe this changes in the future, but with NBC shutting down NBCSN, they are more likely to be getting out of the sports business (other than the NFL) than getting more into it. But who knows what happens five years from now?