I honestly don't think he will leave. I think at most he might miss the first few episodes and then everyone will lose their shit that he has left, but then he'll come back from his little second life adventure.
I hope so anyway. I can't see him killing it and doing that to the fans and the rest of the gang. Because let's face It, if he left, or any of them left, it just wouldn't work that well. They are all needed. It's only two more seasons.
They really are all needed, I mean that's what the Ghostbusters did
I love the whole cast, but the character Dennis is what drew me to the show, and the whole reason I originally made this sub reddit was to have a place to post Dennis quotes. I don't know if there is anything the internet can do to encourage Glenn to finish out the series, but in my opinion they should finish the final two seasons and wrap the series up. It's better to burn out than fade away.