Kubiak quitting a HC job in 2016 for health issues is a major red flag for a guy who will be 60 at the start of the 2021 season.
Gase's future is tied to how Darnold does. The next two years are all about what is best for Sam Darnold's development. They hired Adam Gase because they thought he was a good offensive mind. Keeping Darnold in the same system is a positive. If two years in the system, he is still a below average quarterback, then Gase probably hasn't done his job.
I'm sure Douglas and ownership are aware that the fanbase is very down on Gase, so if their opinions changed significantly on him, nobody would have batted an eye if they fired him.