I think newcomers to the fluff see chaos and loyal marines and no concept of time between the two.
Really, 10k years passed and in that time there are countless marines that would of fallen to chaos e.g. say a squad of ultramarines from 5300 years after the herecy... over that time whos to say if they'd even be recognisable as ultras anymore...
The core legions even employ marines in their ranks from this time period, e.g. they dont just rebel and call em selves somin like 'the darkultraboys' or something
The core legions even employ the creation of new marines from geneseed (both their own and stolen)
So we have:
- 10k years of 'new blood' loyal marines falling to chaos, joining the old legions or their own warbands or even old warbands who were previous renegades or a core legion splinter or anything inbetween.
- 10k of the old legion marines who were in the 10's of thousands, doing exactly the same thing.
- 10k years of creation of new chaos marines from their own geneseed, stolen geneseed and even weird daemonic/warp science creations (just think, you could have a warband of csm entirely founded in the eot!)
All these marines leaving their chapter/legion and joining another or breaking away and calling their warband something else and everything inbetween and constant mergers and warband hoping.
And then to top if off you can factor in the warp/eot time can also be meaningless/bent/chopped/messed with.
All this equals chaos, total and utter chaos.

how it should be.
There is a core of people who cant handle this though, they see every chaos space marine of a specific legion of being 'pure' heh, which is very narrow minded and dull but entirely possible of course for 'their' army but not chaos space marines as a whole.