A little bit of good news, at least: Stan Lee's cameos for Captain Marvel and Avengers 4 have already been filmed. That's according to Feige and the Russo Brothers in recent interviews. Not sure about Spider-Man: Far From Home, though.
It's truly and sadly the end of an era, but I'm glad Stan Lee got to see the Marvel universe unfold so vividly in the MCU. I bet that was something else from his perspective.