It's specifically meant to contrast Simon's reaction to Kamina's death and emphasize how much his character has grown. When Kamina died, Simon stopped moving forward. He barely functioned. He fell into a depression. If he had known what would have happened, he wouldn't have gone through with the battle. But Kamina would have. Kamina would have kept moving forward even at the cost of his own life. He can't keep looking back at the town he left behind, he has to keep looking ahead at what's in the distance. He himself says that all his bravado and big talk would be nothing without Simon, that he was the one who keeps digging no matter what, that he's the one who finds a way. You think that Kamina is the great leader that everyone looks up to, but then you realize that the one Kamina admired the most was Simon. It was this revelation and that brought back Simon's confidence and helped him stand on his own.
7 years later, he's given the choice. He can stop the Anti-Spiral and free all the planets under their subjugation, but it will cost Nia's life. Nia, like Kamina, is willing to move forward for their cause. She will head towards tomorrow. Simon, in the ultimate demonstration of growth, accepts the next greatest loss in his life with grace. He decides that being president of the world wasn't for him (and it really wasn't, he didn't like it) and continues being a digger, helping in his own unique way. But it shouldn't be interpreted as him running away.
The important thing you should take away from TTGL isn't that you can do anything without consequences just by believing in yourself. It's that you can only progress by moving forward, and there will be hardships along the way, but you shouldn't let them stop you from living your life. The Spiral Nemesis is left open-ended because it may be unstoppable, but that's what gives meaning to moving forward in spite of it.