Of course these situations are always complex behind the scenes, with many factors and considerations at play.
But the no he must really just have been fired against his will claim just doesn’t pass the sniff test to anyone paying attention.
If Altman truly is as bad a person as it appears that he might be, that doesn't reflect well on the people who have praised him through the last few decades. If you like those people, then cognitive dissonance forces you to either believe that Altman is being unduly villainized or to believe that the people that you like secretly hate him but just can't say so openly.
The adult conversation in question being, in a nutshell: "We've decided it's time for you to move on. Would you like the public perception of this event to be that it was a mutual decision, or would you prefer to burn some bridges on your way out?"
I think you're falling into the classic reasoning trap:
1. I have realized someone has an incentive to portray the truth in a specific way.
2. They are portraying it to me in that way.
3. Therefore, they are lying.
But 3 isn't necessarily the case! All you can say is "3. Therefore, I can't tell what the truth is." I think that's what people are reacting to in terms of negativity. You actually don't know that PG is lying. You just know that, if Sam was actually fired, PG would have an incentive to portray it as mutually amicable. You really don't have evidence whether or not it happened.