Hi, the main plugin, Newsletter, freely available from wp.org, has no license bindings so you can get updated forever from wp.org when we publish them. Commercial addons can be updated for the time related to your license (we have yearly licenses and three years renewal licenses, right now - but it's subject to change, of course).
All that is clearly stated on the pricing page where is even specified we do not apply for automatic recurring payments, but for any doubts, we are here to support before the purchase (and after, of course!).
That is just a commercial choice: there are marketplaces where you get a license valid forever, there are companies that ask for recurring monthly payments, and so on. Maybe we can change in the future by offering a lifetime license, we've already discussed that.
Thank you, Stefano.