to be fair, Mushoku Tensei is already a pretty good commentary on the mc and isekai readers, though it flies over some of the fans heads. Each of the heroines where a pretty direct statement of the things that were wrong with his personality that lead to his old life being so disappointing. Even if he thought he was going to work harder and have a better life when he was reborn, he had to first learn humility alongside Roxy (coming to understand that no matter how great he is at one thing, there will be others who can accomplish things he struggles with, like just stepping outside and meeting strangers, with ease), comradery/companionship with Sylphie(learning how to make and treat friends to overcome your shortcomings), and responsibility/devotion with Eris (fully committing himself to his loved ones even if it's dangerous and may get him killed).
So they were never the goddesses of wisdom, love, and war as fans say but of humility, companionship, and devotion. Each of them had big events with him in his youth that had a huge impact on his character. He, at the end of the story, was defined by these. That's why the harem couldn't expand beyond the three and why he couldn't try to takeover the world or become immortal with his magic in the end. That would fly in the face of the meaning behind the story and the points made about the character and, by extension, the readers.