Unfortunately they don't let you "buy" anymore. It's a yearly fee a la adobe creative cloud and as soon as you stop paying they "de-update" you a year.
The "fallback" means downgrading. E.g., if you pay for a year now, you get updates for a year. If you don't pay again in July 2017, then you have to downgrade to the July 2016 version, which you may use in perpetuity.
It's designed such that not only do you stop getting new things when you stop paying, you lose some of what you already have.
