"What we really seek in our nostalgic reveries, I want to suggest, is the inertness of the past." Extremely well said. I have never thought of nostalgia in this way but I think it's true.
The fundamental problem is that the way we live in modern society is full of uncertainty that stems from unbridled change. It's driving us all mad because can't stop it, and we don't know what is the next casualty.
Nostalgia is also particularly strong for things we know we've lost and can never get back. Modern life is all about erasing--overwriting, really--both past and present, with new, shiny, utterly cheap coats of paint. Or knocking down and paving over.
it doesn't feel like it is really stagnation if armies are rampaging through your lands and you have new rulers/warlords/famine/pandemics all the time