I had a recent experience with autodesk that shows that free tiers can disappear or be neutered after you've invested significant time and energy into them.
If you just owned that version of the software it would work forever instead you are at the mercy of the platform forever.
It's not just free tiers. Imagine the comments here in a decades time when Figma goes full Adobe and starts to squeeze every last drop of blood out of their customer base.
I don't use sketch, so I don't know anything about it. It's mac only and while i have an ipad pro and an iphone, I don't own a mac and dont want one, and thats a mac only app.
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.
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