Everyone mentions that the first-class folks are seasoned flyers.
But then they complain that they are rich jerks.
The former is far truer than the latter, I discovered when I started travelling all the time for work.
Sure, they most may be decently well off, but not what you'd call Rich People. Most of the people in most of the first-class seats got there on Status. Back when I accumulated about 75k miles in a year, I thought I was pretty hot stuff (it took a while, since I mostly flew domestic). But, even so, on usual "Business Traveler" type flights (think Monday morning, Sunday evening), I'd be number 25 or 30 in line for a first-class seat upgrade.
There's almost never a flight that flies without first class filled for this reason.
Which brings me to my next point...
How much do airlines make based on Rewards programs that will get you upgraded? Sure, the first class folks make up a small percentage of the flight. But by offering those upgrades, and all the perks that go with them (yes, yes, including early boarding), they are buying repeat customers who will fly many, many more times than the rest of the plane, to rack up those miles, to be #3 on the list instead of 30. When you travel that much, you do everything for the points--just like in "Up in the Air".
Just some food for thought on why the airlines might willingly forgo more sane boarding practices.