Often1

Back in the day F pax (way before SM) boarded last so that they would spend the least time in their seat. The sole reason to board early is OH space. Otherwise, I would just as soon board as the door is closing.

If DL enforced its own published security and safety policy regarding carry-ons (and FAA toughened up its enforcement on DL for its failure to do so), the entire kabuki dance around boarding would come to a screeching halt.

My View: Bags which don't fit the sizer (I don't care if they can be shoehorned into the OH) don't get carried on. They are gate checked to baggage office at final destination where the baggage fee + penalty (no waiver for status, etc) is collected before the bag is released. Won't take too many times for those who game the system and now wait in line for 90 mins to cough up a small fortune to get their bag, learn to pack or check at the curb/counter as the case may be. Needless to say, there will still be rare and limited occasions when the OH space is gone. In those occasions, DL can either gate check to the jetway (as it does for strollers) or to baggage claim without fee.

And, boarding can then be a leisurely affair in the order which common sense, not so-called status dictates.