It isn't that paradoxical. Waiting until the last minute can make a you victim of the "overhead bin space" pirates who load up the bins with stuff that should have been checked and leave you to worry about gate checking.
The fear is real. There is no space under the seat for both a bag and feet if you are tall. I carry a reasonable sized carry on but it contains a work laptop and a few mobile devices / tablets (I'm a mobile dev) plus prescription medication. I get so anxious waiting to see if there will be space.
Also, a seated passenger has a better legal status than an unseated one. In the event of a seat dispute, sit down (in any seat, if you have to) and let the cabin crew sort out the mess.
Being seated can't be legally meaningful
Being "seated" is not, but being "boarded" actually is, with respect to being bumped. Generally, being boarded means that you can only really be removed from the plane for inappropriate behavior, otherwise your removal is considered by definition involuntary, which means more compensation (and cash compensation, at that - which I know is changing soon).
And even if it were, I can't imagine actually bringing lawyers into it even in the rare event of the airline not noticing until after boarding that two people have the same seat.
Yeah the real reason to get on the plane first is bag space. With that said I do like Southwest’s method where the earlier you get on the plane the better your seat is. It incentivizes getting to the gate early and as such on time departures and still lets them play with fare classes and tiers
Southwest is truly elite boarding on those few airports where you can board from the nose and tail at once. Feels like we are off in 10 minutes.