If Bevin is no more competition than what Cornyn had, them Mitch is a shoe-in.
You can not drag in some underfunded unknown in the last minute and beat well established and heavy funded incumbents.
Bevin is better candidate than the squirrels who ran against Cornyn. I doubt Matt will "disappear" in the final weeks of the campaign.
But, he is a relative unknown... he's never run for ANY office. So, he had zero name recognition. It's a HUGE hurdle for someone trying to take on the Minority Leader in the Senate.