Having rules, and enforcing them, does regulate behavior. The fear of inconvenience/sizers will reduce overall baggage sizes. Someone shopping for a carryon will remember the bad experience and/or media article, etc, and buy the 20" or 21" instead of the 22" or 23" suitcase. An added fear of confrontation from being forced to check your bag at the gate will motivate some to check their bags if they don't absolutely need the carry on.

1) Average size reduction of 1-2" in carry-on bags
2) 2-3 people per flight checking their bag

This WILL speed up boarding by 5min+ on many, many flights. It is usually the last 4-5 bags that take the longest to check and/or store. On business heavy flights with 20+ too many carry ons, it won't help.

Something is better than nothing! No rules need to be changed, just enforce what already exists. Nothing draconian, a subtle fear of confrontation is enough to modify behavior. If you really really need the carry-on, so be it, you can board with an occasional brief argument. No need to be unreasonable, just add some acknowledgement of those that push the rules (22" carry ons, more than 1+1, etc), and that is enough to change behavior.