This. I-70 doesn't have a lot of spots where the gradient will cause 2WD problems or where the road itself forces loss of momentum due to tight corners. However, when momentum is lost at moderate road gradient (see: Vail Pass) a typical 2WD rental is in trouble.
I pull 2WD DIA rentals back on the road, or up steep sections, all the time on Berthoud Pass. The difference there vs. I-70 is simply tight switchback turns causing loss of momentum and/or sliding off the road. If all it takes is a snow squall and slower speeds to put your vehicle past its drivetrain capability limits, then you are not in an appropriate vehicle.
I don't disagree with anything Bob Barnes said, I simply think the "but I can turn and stop" argument is often used as an excuse to mask obvious deficiencies in forward traction, which is the most difficult area of traction as that is the max vehicle/min driver condition. I really wish people would stop posting that being totally unprepared has worked out fine for them when visiting CO, because it only takes one and it is the wrong message.