You can have whatever opinion you want about the Marine Equivalents in the W40K setting. Realistically, I agree superheavy infantry is not a realistic option in future-warfare.
But the fact remains that if you have "hundreds of thousands" of Marine Equivalents on a single planet, plus their lesser/daemonic cohorts that round out their army, there is no Imperial Guard force in the galaxy that would stand a chance.
...And a single Exterminatus would eliminate every chaos space marine in existence. The Imperium could lose almost any world (excluding maybe Earth and Mars) and still make a net profit if it meant the elimination of an entire enemy faction. Cadia could send most of its forces elsewhere as there wouldn't be any more Black Crusades. Entire chapters could be redevoted to the rising Necron and Tyranid threats.
I speak AD&D to my fluff, 3.5 to my crunch and 5E to my players.