Except they actually can start working on the emulation before they have GTAVI files decrypted: previous GTAs and I think also RDR use the same game engine. Of course there will still be more work to be done after having the decrypted files, but before that you can already get something that can emulate some parts of the underlying game engine.

That may be true, but there is no harm in making sure the emulation is as functional as it can be made before this encryption key problem inevitably gets solved.

If you first wait for the encryption keys you’ll suddenly discover you have yet to make the emulation actually work.