Just go back to the old carry on baggage sizes and enforce them.
The problem in the past was that the rules were not enforced and many passengers took it for granted that they would get away with an extra bag and turned up to check in with that in mind.Most travellers have a regulation(old rules) bag and why should it go to waste, let them bring it on board but no others. It seems that most airlines want to go back to this standard.
I do have a vested interest cos i bought a new carry on just before the new rules, it was smaller than the one it replaced and is actually 2 centimetres oversize on all the dimensions of the new rules.This leads me to believe that the new sizing was deliberatel devised to "measure" all carry on baggage out of the system..