I am not affiliated with the strikers, but I think the idea is that, for now, the companies still want to use at least some human voice acting. So if they want to hire them, they either have to negotiate with the guild or try to find an individual scab willing to cross the picket line and get hired despite the strike. In some industries, there's enough non-union workers that finding replacement workers is easy enough. I guess the voice actors are sufficiently unionized that it's not so easy there, and it seems to have caused some delays in production and also some games being shipped without all their voice lines.

But as you surmise, this is at best a stalling tactic. Once the tech gets good enough, fewer companies will want to pay for human voice acting labor. Unions can help powerless individuals negotiate better through collective bargaining, but they can't altogether stop technological change. Jobs, theirs and ours, eventually become obsolete...

I don't necessarily think we should artificially protect jobs against technology, but I sure wish we had a better social safety net and retraining and placement programs for people needing to change careers due to factors outside their control.