I've flown 15+ hour flights, and yes, I recline my seat. So does most everybody else. To expect someone to sit straight up for that duration, makes you the inconsiderate party.
On a domestic flight within the US, I'll try to accommodate you. Once the flight length exceeds 6, 7 hours, yeah, that seat is going back.
Seats in coach recline and it's not my problem that you're being stingy.