There is a really useful free tier - I think the paid tier mostly includes collaboration features.
If you just owned that version of the software it would work forever instead you are at the mercy of the platform forever.
Caveat emptor
Take a look at Sketch. You can use a file on future versions...but up to a point. If you hit that limit, you will have to buy the upgrade.
I'm not sure why sketch would have the restrictions they have except that forever supporting old formats might be more expensive than they could justify.