You should watch the match when you get a chance, but I don't believe Novak didn't try to fight, and I don't think it was a mental walkabout. In a loss like this, from an ATG, at a Major he has owned, there are going to be several factors. There were several different reasons that Novak lost the W final to Alcaraz, too. As
@MargaretMcAleer rightly points out, above, Djokovic basically hasn't looked that impressive the whole tournament, with the exception of the Mannarino match. He had some kind of cold/flu bug early on, and there had been a wrist issue before, though that didn't seem to come into it.
Your point about the fact that he has almost no goals left to chase is well-taken, and gets to your question of "lack of fight." How do you keep manufacturing intensity when you've basically achieved everything you set out to do? Don't get me wrong, I think he's still going to find reasons to get up for the fight, and add to the resume. But this gets to
@Sundaymorningguy's point above, about age. More and more, the "mojo" becomes elusive, or harder to summon at-will. Like Wimbledon last summer, Novak hadn't really found/had to find his "A" game, and it just didn't show up for him when he needed it. I know a lot of people think that Novak is rather ageless, but he's just not.
Which brings us to Sinner, who HAS actually been having a great tournament. You rightly give him credit for the win. (I know you are just asking for eyes on the match to understand what happened with Novak.) He had a game plan, and he executed. He's got the talent and the chops to beat Djokovic, and he did. You make the point above that he may have wanted it more badly. I think that IS part of it, and good for him for having the hunger. I have always thought that, in the 2008 Wimbledon final, with so little between Roger and Rafa in it, that one of the x-factors was that Rafa wanted it a bit more, having been so bitterly disappointed the year prior. I love that we finally have some youngsters who trust themselves against the greats. Novak was right to say he was out-played.