Like many travel-related topics, this thread seems to have boiled down to an argument over how things should work vs. how they actually work. Sometimes flights board early, sometimes late. Often this can't be predicted in advance -- e.g., the GA scans the assembled crowd and sees lots of children and WAY too many large rollaboards, and realizes they'll depart late if they begin boarding at the nominal time.

As with other dilemmas, you can behave as though the world is perfect and the printed boarding time will never be violated, and then gnash your teeth when you find you're one of the last to board and have to gate-check your bag because the overheads are full. Or you can allow for the uncertainties inherent in any system, leave a buffer, and be the one denying overhead space to late arrivers. Personally I find it most effective to plan my life around reality rather than ideology, but obviously others disagree.