You can save Figma files as regular files on your desktop. My app Flinto includes a Sketch import and Figma import feature. I get your point, but just pointing out its not as locked down as you might have assumed.
Figma would earn a lot of goodwill if they published their specification and assured users and devs in its ecosystem that the format won’t change out from under them. Even then though, they could very easily say one thing and then do another. It’s an unavoidable uncertainty that comes with web apps.