He spent 500 years in isolation, always reminding himself why he's doing it; so that he could be with everyone again. He's not doing it for the world, or anyone else, he's just doing it for a selfish reason of being happy when he could finally meet them again. But somehow the 'reward' he obtained for saving the world is a full memory wipe? Like, that's the worst you could give to someone who's waiting alone for 500 years to meet everyone again. I know author put that memory loss for the feel, but he could have make him regain his memory again to make this a proper ending.
Any of the dungeon are practically impossible to be cleared by a normal human or mc’s hero classmate. Not to mention all of the enemy they face are also too OP for any of his other classmate to handle, making u wonder why are they even summoned in the first place, and how weak are the human in general if they are weaker than the summoned people. Not to mention how the demons are so strong but somehow it was the human that ruled most of the territory while the beastman are hiding in fear, and the demons never launch any attacks before the human summoned the MC. Basically from the looks of it, if the MC does not accidentally become OP the human are practically doomed to ruins no matter how many hero they summon