I suppose it boils down the the question: Are Space Marines what makes 40k? or are they irrelivent to what makes 40k, 40k?
And frankly, if you ask any two space marine players what they thing space marines are like, you'll get three answers each more stupifying then the last. Except smurfs. Every time I go to something like Gamesday, I see more Ultramarines then there are Space Marines in all of 40k at any one time. Millions of identical Rhinos and land raiders. Enough Calgar's to make a Chpater of thier own, which would be dubbed the Sons of Norris.
Personally, I find space marines to be irrelivent in 40k. Other then as things to explode like melons when hit with a Basilisk. Politically speaking, in all honesty, if the the SM and CSM had annihilated each other at Terra, another body would have risin to take thier place in the galactic power vaccum. (Probably the Adranti or the Diasporax, both of which being advanced starfareing organisations.) with probably broadly similar results. (Though the humans might not be on such rough terms with xenos. Or, in other words, the changes of 2nd and 3rd ed fluff might not have happened.)
