Most likely you won't even notice some of the changes. Reasonably believable cars can already be added to films in post, so no reason why humans couldn't be. This might not be driven only by AI, but instead on more general tech developments in vfx and such.
Really, extras have been being replaced for decades with lots of ways. You don't need to fill a stadium with people, just use cut outs. Or CGI in an army instead of paying for a lot of people and costumes. This is the next logical step
There's a technique called 'tiling.' You hire, say 100, extras and have them move as a unit from one part of the arena to another. Mix them up so people don't see the guy in the green hat sitting next to the redheaded woman all the time.
That's one reason I like the pre-CGI movies. When you see a crowd or an army, you know it's real people.
Two movies I recommend to everyone. 'Waterloo' with Rod Steiger. To make 'War And Peace' the Soviets trained 16,000 Red Army troops in old time cavalry, artillery, and infantry tactics. They later rented those troops out to Hollywood. The battle at the end of the movie is one of the most authentic reenactments ever filmed.
'Thunderball' with Sean Connery. So, James Bond tells the CIA SPECTRE is smuggling stolen A-bombs underwater. The Navy paradrops 100 SEALs into0 the middle of the ocean, where they put on scuba gear and fight bad guys armed with sea sleds. All real guys...