If you keep developing it and charge for access to a privileged access to updates, I'm sure you're sales will keep up. If you're updates are worth it, that is.
People won't pay for it because all of my hard work and updates are immediately shared, for free, by someone that bought it once (allowable by the GNU license). You aren't allowed to restrict the user in the form of a proprietary license.
"f you keep developing it and charge for access to a privileged access to updates, I'm sure you're sales will keep up. If you're updates are worth it, that is."
As long as you are using the GNU license, these changes can be shared for free.
This is why services are the way of the future. No piracy, updates are easy, and you also get to charge a monthly fee.
I also think its crazy that people send you hate mail. That right there exemplifies the sense of entitlement they have. Don't get me wrong, I love free software, use it, and I release code occasionally but I'm not about to starve because Stallman says I have to choose either or. Free or proprietary, you must choose one is the message out of the RMS camp.
The problem is that "free/gratis" and "free/libre" are intertwined. If you sell an app to a user that is under the GNU license, they have the right to re-compile and re-release it for free (and it's allowed under the license).
I've tried to charge for software under the GNU license. The type of people that use it generally expect it free as in beer. If it isn't, you get hate mail and someone will eventually release their own free version (which is an exact copy of the the paid edition) to compete with yours. You will slowly lose all of your sales and traffic unless you also release it for free.