I interpret that as Hillary's inevitability being unshakable from the start, regardless of who her opponent(s) were. Trump could've been in trouble with a strong unifying figure, but it's doubtful such a figure could ever have materialized, and same goes for the Dem side. Candidates who tried to be all things to all people tended to flame out early or put in embarrassingly pathetic performances (Walker, Jindal, Paul, Rubio, O'Malley.)
It's less clear to me that this is telling us about what might have been, as opposed to how supportive the parties are of their nominees now.
I mean, 45% say they're satisfied with Trump as the nominee. But in an alternate universe where Kasich didn't run and Rubio didn't make a debate gaffe, a larger %age than that might have been supportive of Republican nominee Marco Rubio. Given that Trump started the primary season with a bunch of 35% plurality victories, I don't think it would have necessarily taken a "strong unifying figure" to beat him. Just a somewhat different field of candidates and/or strategies for beating him.
