For a realistic look at Flint and Moore’s relation to it, I recommend the book “Rivethead” by Ben Hamper. Hamper actually worked the assembly lines for years (unlike Moore), and made it clear that Flint had been in a slow decline long before the events of “Roger and Me”.
As to Moore’s fading from relevance, he irretrievably associated himself early on with the blue-collar voters that the Democrats have since thrown aside in favor of identity politics and urban snobbery. Witness his speech describing Trump’s election as “the biggest eff you in recorded history”...it explained the Rust Belt’s votes, but went ignored.
Hamper described Moore himself as a lifelong troublemaker who stood out as a “radical” (posturing or otherwise) in blue-collar, Catholic Flint. Once he headed off to the big leagues at Mother Jones, Moore realized he was a small fish in the huge California pond, and kept looking for reasons to come back to his old BMOC grounds in Flint...needless to say, his bosses at Mother Jones didn’t appreciate his absence from the office.