Let's not trick ourselves here. In a regular race this would be a tight campaign no matter what. The economy is good but not strong, foreign policy has had successes but still worries Americans, and the Incumbent is popular but divisive. The fundamentals, as Nate Platinum has said, strongly point to a tight race.
And I heard many saying during the Cruz/Trump/Kasich deathmatch that Ted Cruz was more hated by the Republican Establishment then Trump. Ted Cruz was not a winner until Trump built up a base that Ted Cruz could leech off of. Cruz's plan was to ride Trump's coattails until Trump self-destructed, then sweep in and take his supporters. It failed because Trump never self-destructed, but if Trump was never there in the first place, well that it would depend on Cruz managing to build it up in the first place. And people loathed Ted Cruz. The establishment loathed him. They didn't forgive him for the shutdown. He'd be painted as a megalomaniacal politician masquerading as a principled right-wing crazy. And MSH, you've forgotten that the shutdown was very unpopular, it just wasn't in an election year. Bringing the guy responsible for it would have hurt as Americans would have had an opportunity to remind themselves.
Plus, after all, Trump deserves some of the credit for the Hillary hate. Not all, by any means, or even most, but if Ted Cruz was the Republican Nominee then Wikileaks would never have happened, the hackings would never have happened, emails would never have happened, Trump bringing up everything from Bill Clinton to founding ISIS, and then a lot of stories play out differently. Counter-factuals are, by definition, hard to figure out, and claiming that its obvious what would have happened does the truth no justice. People forget what it's like to oppose regular Republicans just because they aren't Trump. Trump has polarized and lowered the bar so far that people don't believe anything. Who knows what gaffes Ted Cruz or Hillary makes in that timeline, or what zingers get off, or all sorts of things.
Plus, let's not overstate Ted Cruz's brilliance here. He refused to endorse Trump on the convention floor, then endorsed him a week before Pussy-gate. He definitely could have played this more intelligently then he did.