Hillary is not a lock.
That said, I do agree the first debate could be a great breakout moment for her. The ratings will be sky high, and the contrast between the two on the debate stage should leave a lasting impression on people who want a serious president.
Debates nowadays rarely move the needle - the tend to just reinforce beliefs. I actually think the conventions, back to back, will have a huge effect before the Olympics. People will be able to contrast the Republicans and Trump's VP pick with Clinton and her's. I also think having Obama campaign for Hillary after the primary will be a big lift.
Operationally, Trump is so far behind in his ground game and data analytics that he is already in a deficit. He really thinks he can win on sheer force of personality and social media.
Kerry and Romney both got pretty decent bounces from the first debate.
I agree both that the debate can matter to a mild degree, and that Trump's supposed decision to dismiss turnout analytics is colossally stupid and will almost guarantee his defeat. I don't think Republicans can win in a Presidential year without them at this point.
