With respect, I really don't think a comparison between older-gen games and 'current' can be made without considering the different amount of graphical content contained. Newer games take up so much computational power to show off the tracks and environment.
The devs of newer games are also operating in a completely different era of business practice; one that affords less freedom to experiment with and they have more financial constraints. This is why stages tend to be shorter as well as being extensions of other stage sections I think. It's a technique they used in D2 too and that seemed to work just fine. WRC3 has longer stages but graphically the demands put on the system are far less.
I'm not defending all of CM's choices though, even if I do like all three DiRT games a lot. Some choices CM made I am not happy with, but accept in the long run. Some would say that by accepting, we are giving the devs a license to do what they want. Considering the amount of feedback and complaints I've seen on the CM forums since D2 plus the fact that nothing changed, I'm resting my case: they do what they can with what they have.
D3 online I think died because of the usual track cutters and rammers getting away with what they do, plus the 'any vehicle, any track thing'. Hardcore was a good mode but died first because those rammers and cutters couldn't do that in hardcore, except in the rallycross sections. I never worked out why that mode was included in hardcore, not without collisions disabled.