Magic immunity is a boring power. The game is about things happening. Blanket immunities make things not happen and the game not progress. This is something that should be done sparingly, and should absolutely not be the basis for half the classes. It's also unfun, because it means that lots of encounters involve people's abilities getting noped and them functionally having to sit out the fight.
Also, yes, it is absolutely true that this does nothing to allow Fighters to solve problems, which is the fundamental goal of adventuring. If you are 99% immune to Wizards, but can't kill them, you still lose.
How is that different from "tell someone who posted something you think is bad that you think the thing they posted is bad"? What I am supposed to do if someone suggests a change I don't think is good? What is the point of asking for feedback if your response to negative feedback is just going to be "that's not what I'm trying to do"?
What "cool martial based stuff you see in fiction"? None of the moments from fiction I would describe as "cool martial based stuff" are people being immune to magic. The mode the fiction supports for balancing martials is giving them magic. Aragorn gets to be a Ghost King. Thor and Arthur get magical weapons. No one gets "total immunity to magic".I think the guy is trying to recreate some of the cool martial based stuff you see in fiction. That's not exactly unheard of.