It's not a subscription - you can choose not to renew after your upgrade license expires, and you can still use the software fully, and you're not nagged. You can choose when, or if, you get a new license, based on if you need new features. It's up to you.
Compare to buying a traditional software package, where you buy one version of the software. A new major version comes out with new features. You can choose when, or if, you upgrade to the new major version, based on if you need new features. It's up to you.
These seem to be pretty equivalent to me - the reason we're going with the former is we don't plan on having "major updates". We'll just be releasing regular small updates indefinitely - a bit like Google Chrome - rather than infrequent, big hefty major updates - like Internet Explorer.