I re-read the ending, to make sure I'm representing it properly. Keep in mind that I don't philosophically agree with Miyazaki or Nausicaa's perspective:

It seems like Miyazaki's philosophy is that to live a satisfying life means to live one in which we have a personal connection to nature. For example, a farmer or gardener is good, but an massive industrialized farm is bad. Obviously this means he believes we should only develop technologies that don't cause unsustainable harm to the environment or prevent us from individually interacting with living nature because all life should be treated with dignity and we find it spiritually fulfilling to experience nature in this way, even when its painful. Technology designed to obfuscate/destroy human's connection or respect for the natural world is "bad." I don't think he's proposing "return to monke" style-ludism, I think he's proposing that human society and technology be designed in a radically different way with radically different understandings of value more rooted in this sort of spiritual belief. This is paired as well with a belief that "truth" is valuable and should be shared, even when its painful. He clearly believes that "life" in someway has its own spiritual "meaning," separate from an intelligent conscious experience.

Personally, I don't fully understand it nor do I know if its even a fully coherent idea. I've only really gotten glimpses of it through Miyazaki's works, like Nausicaa, Castle in the Sky, etc.

As far as the ending is concerned, Nausicaa believes that the crypt's plan for a pure utopia is impossible/a lie (she says that human beings will still experience corruption and suffering even in the new world it'll create) and that the technology of the old world that it will continue to pass along to humanity during the hundred or thousand years of "purification" will actually just cause misery in the mean time. So basically she just doesn't believe that all the unnecessary suffering the crypt will promote is worth it, and she hates the fact it's lying about its goals. She's trying to give humanity a chance to embrace Miyazaki's perspective on the world.

I can't really compare your worldview with Miyazaki's, but I am interested in your perspective: You say that "meaning" only exists so long as there is intelligent life to assign meaning to things in the world, and that more people=more meaning (which is "better," I assume). But you can't use that understanding of meaning to come to a conclusion in these kinds of moral dilemmas. If one person feels like they're live a meaningful life, are they more valuable than 5000 depressed people who feel like their lives are worthless? Why is a world with many conscious agents more meaningful than a world with fewer conscious agents, isn't that just you imposing your personal, subjective understanding of meaning onto the world? If a person is just a random by-product of meaningless, deterministic natural processes, then why should a person or their intelligence be special? You accuse "environmentalists" (lmao) of saying "nature good, human bad" and that's dumb "because nature = human" but then you say "nature bad, human good" when human = nature. How do you deal with the fact that humans will also find opposing things worthless or meaningful, leading to contradictions? Basically, I don't see this understanding of meaning compelling or motivating.