Plugins encode their state as a combination of automation parameter values managed by the DAW and opaque binary blobs for everything else.

In the general case of professional-grade use, you can't assume that two versions of the same plugin will handle any of those in compatible ways. In some cases they might, but there's nothing that guarantees it and many known cases where it doesn't hold.

And speaking to your broader point of "there has to be a way to translate between the two" -- this is likely true in a formal sense, but not a practical one. If it was 2030 and your job was to revive some dead AAX plugin and open its projects in some new one that you were writing for AUv4, you could do a whole bunch of bespoke debugging and forensics to make it happen. But there's no solution for the general case, as applies to a converter like this.