The seats are assigned. People have been standing in line for 15 minutes now. Why on earth would anyone want to stand there, when they could just sit and wait until the line clears?
I understand wanting to get off a plane ASAP, but boarding? You just end up sitting on the plane, waiting for everyone else to get on.
If you have it in a backpack you can put it under the seat in front of you unless you’re in a front seat or one next to an overwing emergency exit.
This is all officially allowed and I’ve used it plenty of times.
Bags on the floor take what little space there is for my feet.
Once I’m in the air my backpack goes from under the seat to under my knees. Then I can stick my feet under the seat for that extra few inches of stretch. It’s not a whole lot but it does help.
True, it can be a tradeof, depending on the length of one’s legs.