Well on the table top, Chaos Marines who are 10,000 years experienced have the same stats as marines
so I am not so sure about this.
Because every CSM is a survivor of the Horus Heresy, who has been fighting for 10k years and wasn't killed.
Anyway, to the OP.
Space Marines are Space Marines. They are just as tough now as they were then.
Except Calgar and Dante. Apparently these 2 are just freaking beasts.