Yeah, I probably could just do weightlifting and cardio on their own (and will sometime have to transition to this) but to be honest football is my primary motivation for doing these things in the first place. A lot of competitive athletes make a successful transition from competitive play to less injurious activities after their careers and are none the worse for it. (Minus NFL concussion cases...)
There are also intangible benefits to playing competitive sports. Being part of a team, working hard to achieve a goal, being challenged, pushing your limits, pursuing something you're passionate about, succeeding at an elite level... Sports can give you unique experiences you'll treasure for your lifetime and also shape you as a person.
It really is an optimization between the all the trade offs. Every year I have to weigh what the sport gives me and what the sport takes away and decide whether it's my last season.
In fact, it seems it does. http://www.scientificamerican.com/article/does-heading-a-soc...