GOP turnout and GOP vs Dem turnout numbers are trending parallel to 2000...the year the Supreme Court decided the election. You put Cruz at the top of the ticket and you really think those folks are going to sit at home? They'll get out and hold their noses just like so many Republicans will when their guy doesn't get the nod.
Cruz is trending to what could be below threshold in Florida. Rubio dropping out could put Trump above 50% (needing about a third of his votes at most) there and effectively neutering Cruz's claim that he has a high floor and a low ceiling. This claim that a one-on-one will go against Trump has no legs...