This isn’t particularly a new issue. Once an app is compiled, that’s the end of the authentication check. You could at the moment subscribe to a pay-monthly plugin, compile your app, and then immediately end your subscription. Your app would still work as an app indefinitely despite not paying for the plugin any more. If you later want release an update you’d have to subscribe to the plugin again and compile, at which point Corona would drag in the latest plugin version.
With offline builds it would still be possible to subscribe to a plugin, download the files, and then unsubscribe. Granted with the new approach you’d then be able to build as many times as you like but still, you’d have to resubscribe to download the latest version of that plugin.
Personally as a plugin vendor myself, I think a plugin should only really warrant a monthly charge if the buyer gets something every month for that charge. Plugins updated often warrant this, and plugins that provide a service which consumes remote resources such as chat room functionality warrant this. Again updates wouldn’t be available without the maintained subscription so that example is covered already, and for service based plugins my advice would be to build activation keys in to that service and charge a monthly fee for those keys rather than for the plugin itself. This just seems like a better modal to me.
For plugins that really are just a one off download that adds functionality that’ll continue to work indefinitely without any communication with remote servers, I honestly don’t see that a monthly price is fair. Value that plugin appropriately for the amount of work that went into it, divide that value by the number of developers you’d expect to grab it, and that’s your fair one-off price.