The one thing that letter does do, is poison the media tour for Guardians 3. If Disney doesn't rehire Gunn, the media tour will be 100% about Gunn, you won't be able to block those questions. The easiest answer is probably simply, don't do GoTGv3 without Gunn. But...The Avengers 4 press tour will also become all about Gunn because of this letter since the movie features the Guardians.
The cast doesn't seem like they should have a lot of power, but they have really boxed Disney in. They can't just fire and walk away now, I think the Ringer article is good at expressing that.
My gut says there will be some form of compromise position that gets announced. Gunn will be able to come back, but new Twitter/Facebook rules will go into effect for Disney creators or he will come back as an Exec Producer/Screenwriter but not the Director.
Disney is going to find it harder and harder to find talented Directors to take on big franchise movies if it doesn't figure out a better way to deal with Twitter/Social Media.