I think so many things made it "over for Trump" that would've ended any other normal politician a thousand times over in the last year. They didn't because for so long we all treated Trump as a mere novelty act, something transient which would end with the primaries, so it was all just taken as par for the course and everyone assumed he would disappear from the race soon. But when he won the nomination, our press had no idea how to handle it.
They lazily tried to deal with it by hoping to refashion him as a normal Republican candidate, especially when he was in bad, bad shape in July and August - they saw the ratings potential dissipate as Hillary began to dominate, so they tried to give him a dozen chances to pivot and run a 'normal' campaign. But they realized too late that he couldn't and wouldn't be remade into a normal candidate. They continued trying to desperately pursue this same old both-sides false equivalence narrative with the two candidates until the last possible moment - namely mid-late September. When the worm turned it turned hard, and that's where he has been since, as a rolling disaster. Everyone knows it, he is cratering and Hillary is fully in control.
I think the debate, Alicia Machado, the tax leak, this tape - those are the things that have really destroyed Trump once and for all. There will be no coming back and there will likely be more scandals, but those are the things that officially sealed him off, after a year of too many of us shrugging at it because we thought he'd never get this close. And that happened because of the blinkered, myopic GOP.