Yep, Southwest get it right again, you don't get to encroach on somebody elses seat space, pay extra for your own space.
So in the same vein, my seat is designed to recline and so the space behind me that it reclines into has been allocated to me for the duration of the flight. If you are sitting behind me and have long thigh bones you should purchase the seat next to you and extend your legs into the legroom there. Having an empty seat next to you is a very effective way of improving comfort in economy class and not as expensive as a business class seat. You have many options.