We already have undetectable fakes. Anybody can buy a blue check mark on Twitter and pretend to be somebody else. Anybody can also domain squat.
The way we detect them is if this becomes a problem, the authentic entity will speak against it and the society will rectify. Just as these fakes are a non-problem in practice, AI fakes will be a non-problem too.
It's not exactly the same though. Attacks from spam networks out of extradition reach will become more sophisticated, and that's going to take more money or more annoyance to slow down. The same way phone call scamming from India is a billion dollar industry, fake ads impersonating famous people and things like that will have near zero cost to produce which means it's cheap to mass exploit.
Much cruder fakes than you are describing are a huge problem in practice, costing billions per year in fraud to Americans alone. Just because you can detect them, doesn't mean your grandma can.
The societal implications of undetectable fakes are off the charts.
People are saying this. You're on HN though, the demographics here can tend to skew money-grubbing techbros with who is the loudest and most defensive of anything they've got money riding on. Remember the crypto and NFT hype on HN? The bottom-line is only what matters to a good chunk of people here.
Problem with this for example is when LE uses it to incriminate you, and forces you to prove that their evidence is fake. We can see from the Horizon scandal that when they want to keep you from defending yourself, they usually succeed.
The way we detect them is if this becomes a problem, the authentic entity will speak against it and the society will rectify. Just as these fakes are a non-problem in practice, AI fakes will be a non-problem too.