Bush will lose regardless of what Trump does.
Yes, Bush will lose , but he would also mean a Third Party run by Trump.. Better to nominate someone else to begin with.
That is perhaps a long-winded way of agreeing with your statement. Just as Teddy Roosevelt outpolled my fellow Ohioan in 1912, where Taft was the Republican nominee and Teddy ran on his own ticket; a Trump Third Party would out-poll a Republican Jeb Bush ticket. We have had a preview of that fact over the past week.
That said, I hope the analytically challenged "pundits" in the Republican establishment in Washington, will not continue to play stupid, and drive this potential very, very serious asset, away.