For those that fly on airplanes, the overall experience is at best, tolerable, and at worse, it's just plain bad. Not too surprisingly, people want to try to do what they feel that they have to do to better their experience, of which, one of those things that some people insist upon, is early boarding, of which, we do so find that there is a certain percentage of those people that are apparently so eager to get onboard the airplane that they are willing to even pay extra just to board early.
While there are indeed some airlines, that necessitate early boarding in the sense that they do not pre-assign seats, such as Southwest Airlines, most airline carriers provide the passenger with a specific seat assignment. So then, for all those that have no carry-on luggage and no little kids, there doesn't seem to be any legitimate reason to get on board the plane, earlier than so necessary. Also, for those that do have carry-on luggage but know for a certainty that they can stow their luggage, such as a backpack, under the seat in front of them, there surely isn't any good reason to board early. As for those that know that their carry-on luggage must be stowed in the storage bin, that is above them, there is then a reasonable concern that perhaps luggage space will run out for them.
So then, for all those that are quite concern about overhead space, the very first thing for them to do is to find out through the agent at the gate, how full the plane is, and if it isn't real full, then not to worry about finding space. The second thing for them to consider is that people are entitled to stow their luggage wherever it will fit, so that a good plan to have when a plane is quite full is to stow one's luggage in the first available bin that presents itself, on the way to one's seat. Sure, there may well be a situation in which all the bins are full, and perhaps that necessitates some degree of prudence in boarding the plane, but to pay extra to early board seems like an unnecessary expense.
Further to the point, flying on planes is uncomfortable, and those that board early are then sitting in their seats, a heck of a lot longer as compared to those that are still in the airplane lounge area, in which, these people have plenty of room to spread out, finish up tasks, and just be comfortable. The rush to board any plane, just seems insensible, valid excuse or not, because as long as you are in the line, before the departing gate closes, you will be allowed to board. All those then that complain about all the contingencies that could go wrong that therefore necessitates their early boarding, need to recognize that this is true of not just boarding airplanes, but life itself, and virtually nobody spends their time in a rush to always be the early bird, when the meeting time isn't going to ever adjust to their early arrival.