I'm going to have to start my explanation with what Gurren-Lagann was about. Gurren-Lagann was about, at its very basic levels, progress, and how it is the job of the young to surpass the old as time goes by.
You can see this at multiple points during the story, and hear (read) it plenty of times in the script. Particularly in episode one where Kamina remarks that the youth of the village will break through to the surface. It also comes up in episode 27, where Simon gives his speech. "The wishes of the defeated and the hopes of all who follow; with those two thoughts, weaving those aspirations into a double helix, we'll carve the path to tomorrow!"
You seem to think that Simon wasn't a digger just because he was glad to escape his village. It wasn't escaping the lifestyle of a digger that made him happy. It was moving forward. Making progress. Simon repeatedly stated that he was a digger. He enjoyed digging, when he made his speech in episode 11, he called himself "Simon the digger." Digging was a part of him, you must have misunderstood that.
Your next concern was that they could just revive Nia, but you missed the part where Yoko told Gimmy that Simon was not a god. Sure, he could do it, you know that. The Anti-Spiral also knew that. In-fact, that was his motivation as a villain. He was trying to stop the spiral race from abusing spiral power and destroying the universe. Simons final words to the Anti-Spiral, as the the Anti-spiral Told him to protect the universe was "Of course, Humans aren't that foolish." Reviving Nia would be foolish. Nia's death was not useless to the plot, it was a powerful way to show that humans knew not to abuse spiral power and end the universe. That's a really really important lesson.
I hope now you have a better understanding of the show. Sorry for any grammatical mistakes. I'm not going to check.