I don't think it's wishful thinking necessarily, the Judge Curiel thing had a pretty noticeable impact on the race. The difference there was that it involved a Judge, who obviously didn't get involved and let it die, to be forgotten once the email story blew up. Khizr Khan is, thankfully, free to go on TV and continue egging Trump on; Trump, being who he is, seems incapable of not taking the bait. Now Khan waving a copy of the constitution at Trump is being run on a loop on the cable news networks.
I doubt this will sink the Trump campaign once and for all, but I think it is a strong indicator that this campaign is doomed to failure. Trump has somehow managed to get the most effective attack launched at him in the entire 4 days of the DNC to be played over and over again, for who knows how many millions of people who otherwise might not have seen it. And yet he tweeted about it again this morning, ensuring that instead of dying over the weekend, the story now makes it into a fresh weekday news cycle.
Quite simply, if a presidential candidate insulting the religion of a gold star mom and questioning the integrity of a dad whose son died heroically serving this country is not something that is going to turn the electorate off to said candidate, than I don't recognize the country I'm living in. And this as the kickoff to the Trump campaign post-conventions.