I see only one reason, why i would want to be early at the seat. its bcs if i am not, my backpack might be placed above but multiple seats away by the crew, where it is then uneasy for me to have an eye on it whilst easy for theives to take and open them, especially on long flights there would be plenty of opportunity like when everyone is sleeping.

but for this case i use locks on the backpack anyway, so that anyone who wants to open it, either opens it where nothing of value is in it thus no lock, or at least has a much harder time than when trying the very same with other bags...

also on longer flights i usually did not have that problem, but that could also have been just luck

The question you should be asking instead is why would you shoot a man before throwing him out of a plane?

I literally chill til the very end of boarding before I get on. Like you said, my seat is assigned, it ain't changing, and I'm packed light enough to put my backpack under the seat in front of me. Sometimes people don't show up n I sneak into a better seat 🤫. I've only ever been called out for that on Frontier, they know the hustle apparently. I've never bothered trying 1st class or anything, but I've gotten some middle seats to transform into aisles that way.

eh, if youre in first class you could potentially start getting served drinks before boarding is complete. i know i have.

that said, ive often waited til the plane was mostly seated/no line before boarding even if seated in first.