This is bold talk from a one eyed fat man (OK I reckon Rove has good vision). They know Trump can go 3rd Party just as easily as stay with the GOP, and they’re scared shiteless he’ll do it an leave the R party in shambles with a fraction of the vote and either Trump or Hillary as president. So they won’t do it. Not because they are smart but because the senators and congressmen who depend on the GOP money-raising machine won’t let them.
That is true right now, but wouldn't be true after the convention. There are at least 12 states that have "sore loser" laws that would prevent Trump from being on the general election ballot as an independent after being on the primary ballot for one of the parties. And the filing deadlines for a lot of other states will have already passed by the convention. So if Trump went into the convention with 35% of the delegates, for example, but lost the nomination in a brokered convention, he would not have any way to run as an independent and win. Now he could run as an independent in some states, but only as a spoiler to make sure Hillary won.