No matter how good or bad this is dealt with, never forget that MC was braindead for years before story started and abandoned his friends to do "work" that nobody expected of him that gained him nothing because author bullshit reasons.
Uhm. Nah dude. Not how it was.
The thing is that MC was under orders straight from the King himself. As the court magician, his job was to support the Prince in gaining achievements through dungeon crawls. There was no way for him to reasonably back out until the Prince fired him.
I mean, look at how the Prince was behaving. And then at what the King says to him. It's obvious that in this setting, that King's power is absolute. You do what he says or else you'll be penalized. Fortunately for him, he's foreseen what will become of his country (Read: Him) if it continues on like this. So now he's calling for reforms...
Though that seems too little too late, judging by the Prince's behavior.
Besides, I'm pretty sure the MC had another reason for sticking it out. In the first chapter, he mentions something about proving it to "that person". (And I'm pretty sure "that person" wasn't the Prince.)