if companies are making AI voices for siri and alexa sound realistic, hypothetically the AI voices can slowly replace voice actors, amirite?

Hypothetically is the key word, maybe one day, though AI doesn't always sound the most natural or come across as trustworthy. Pronunciation aside, I have yet to see AI that uses tone well without being based off of a voice actor.

Screen Actors Guild would have a giant say in that matter

AI is probably the next order of evolution.
If we don't destroy the world first, robots within the next hundred years or so will be able to perform any  physical or intellectual function better than any human can.
And they will be able to self-replicate or repair as necessary.