They’re not giving up though, what they’re doing is getting ahead of it. Assuming their deal is favorable for their members, they’re making it so that anyone who wants SAG-AFTRA synth voices has to go through their contracted company which they have collective bargaining power or strike an equal or better deal. Along with blacklisting companies from SAG-AFTRA work that use non-union synth voices.
This is way better than leaving actors on their own to bargain with companies, which would have definitely happened. Rather than have companies wear individuals down and drive pay down, they get to dictate the terms, together.