The key word in this argument is "might". Of course, Matthew McConaughey might have accomplished more for society had he attended law school. He might have gone into politics and maybe would have become an influential figure in congress or even in the White House. Of course, he might have failed or become a second rate ambulance chaser, but he might have done well. Yes, he might have.
It's hard to say what might have happened if McConaughey had become an attorney. Some attorneys do good things for society. Others do not. Some attorneys get involved in corruption and end up stealing from the people they are supposed to represent. There's no way to know if he would have done good or bad with a law degree.
If law wasn't his passion, then he wouldn't have excelled at it the way he has in his movie career. I think McConaughey is a great example of how anyone should be allowed to follow their passions. He's successful and he's happy and that's how so many more people could be if they just studied what they wanted to.
Matthew McConaughey was not passionate about being a lawyer, for one. He would not have done as well. Not only that, but lawyers are not necessarily a benefit to society. His lack of passion would likely have led him to becoming a low level lawyer or someone who focuses on real estate or tax law.