If you find the time to continuously add features, maintain the plugin, and provide support for your users, all for free – you are really awesome and please keep doing that! But that’s not the usual case, and if you want to spend more time improving your plugin and providing great support, you’ll have to find an additional source of income for that time. Doesn’t it make sense to provide ongoing support and new features in return for a recurring subscription fee? Everyone will be happy — your free users will get a better supported plugin and your paying customers will be happy to pay for the extra features in the premium version.