But assumptions and suppositions don't equate to proof of immaturity. Even the example of McVay saying there was a communication breakdown and his admission that he could have handled things differently don't illustrate someone who is immature. If anything, it's an exhibition of maturity.
Now, McVay obviously can't be incredibly experienced at this point since he's so young but I haven't seen any proof that he is immature. It's been the exact opposite IMO. With Goff, he didn't make a rash decision based on emotion. He gave Goff at least two years to work out his problems. Obviously, once McVay was "done", things changed quickly. But McVay and Co. don't fool around when it comes to guys who they think don't fit anymore. But again, that doesn't suggest immaturity as much as conviction and decisiveness. And, as far as I can remember, at least with the bigger names, they haven't jettisoned anyone who went on to bigger and better things.
This whole maturity conversation seems to me to be a false narrative propagated by those that are upset and disappointed about Goff's departure. IMO, McVay did not want to get rid of Goff. He likely did everything he could think of to get him back on track but Goff just couldn't acclimate to how defenses evolved. IMO, Goff's limitations have cost the Rams a realistic shot at two SB rings. His weren't the only shortcomings the Rams had, but they were the most important IMO. The things that he struggled with all flowed downhill, leading to other problems. It got to the point that McVay is being looked at as immature, tyrannical and ineffective.
I don't blame him for wanting "his" QB under center. At least that way, if he fails, he fails on his own terms. Having to "fix" Goff in addition to SB expectations was just too much.