I'm actually not against their origin fluff. Nor do I view it as ridiculous nor altogether infeasible, at least no more so than anything else present in 40k. They fit into the lore well enough. I don't like how mini marines are seemingly being cast aside and replaced, but primaris aren't inherently bad. I think people are just butthurt because its a major change no one got to see coming. GW pulled a fast one when they introduced these guys.