We, the people, need to shift from the idea of “we’re buying plane tickets” to “we’re renting space on a plane.”
My purchase includes:
my seat
all space to the ceiling
all space between my seat and under seat in front of me
Airlines then have an obligation to protect my rented space, since that is what they sold.
This happened to me on Delta.
It's not all that unusual but on one particular day, the woman seated next to me was so obese that she took up approximately half of my space. I'm not exaggerating.
This meant I couldn't put down my tray and I had to let her use the whole left side of my seat and the arm rest, and I squished over to use only the right half of my little seat for a pretty long flight.
I didn't want to embarrass her and hoped the flight attendants would make some adjustment and they definitely saw, but they pretended not to notice.
I took a discreet photo and asked Delta for consideration on another ticket, say half price, since I only got half a seat LOL, but they told me there was nothing they would do.