I would not expect CO, or any of the US carriers, to start strictly enforcing the carry on bag rules. The dirty little secret is that they prefer as few checked bags as possible and would rather folks cram as much in cabin as possible

Checked backed are far more expensive for a carrier to handle than carry ons. With carry ons the passenger is schlepping their bags for free, with checked the airlines have to pay folks to do the schlepping.

It's no surprise that carry on rules are not enforced.