He pretty much died with the thought that Thanos won, and never got to see the Avengers overcome that loss in the most impactful way possible! Even though he was aware that Endgame would follow up from Infinity War, it’s the fact he never got to witness it or even knew the details on the story that makes it sad.

It would’ve been the perfect final Marvel movie he’d get to see before passing.
Whenever I think about Endgame on occasion, it sad to have to remind myself that Stan Lee didn’t get to live long enough to see it!