The flaw behind your counter-argument is that because most people are decent, all are. This is clearly untrue. There needs to be a legal means of stopping commercial rip-offs, and this, in my view, is the reason why copyright exists. My own e-books are explicitly sold on the basis that more people are decent than not -- look at
http://www.rogerandfrances.com/e-books.html where I say,
There is no attempt at protecting that file, because we are relying on people being honest. Some people are going to steal it anyway, out of thoughtlessness, meanness or malice, but we believe that most people aren't like that, and besides,our prices are extremely affordable. -- so I am not misanthropic.
In fact, as you can see from the above, I am putting my money where my mouth is. Bear in mind that for you, this is a largely theoretical consideration. For me, it's a matter of income. I have therefore thought about it quite hard.
In other words, there needs to be SOME way of stopping someone exploiting my work on a commercial scale, or selling it for less than I sell it for. If I charge $9.99, to cover all the work I have put in to it, and someone can rip it off and sell it legally for $0.99, having put no work in to it except stealing it, you may rely upon someone turning up who will do so. In this context, some form of copyright protection is an indispensable back up.