Most of the work going on with rendering people speaking isn't to defeat captchas, so you can't use the speed of that work to judge how fast people will be able to work to defeat a captcha. Also if it were in use people would frequently change the kind of actions they are asking for.
>Real time-ness and a good deal of rendering fidelity can be mitigated by pretending to have crappy network connection once the prompt is given,
Real timeless and rendering fidelity are the only part about this that isn't solved, so there's no real debate to have if those constraints aren't there.
>unless you are comfortable excluding people without reliable internet.
If this was a real thing, I will absolutely bet that companies would exclude people with unreliable internet once I becomes known that all scammers are pretending to have unreliable connections.
"Please find a more reliable connection, or stop by the nearest Identity Check facility to authenticate yourself."
>I think the bigger issue is probably with the premise though.
I don't think this is likely to happen either. I think the most likely solution is trusted third party in person verification. Where I could see something like this working is if the third party verification service offered a 1 time remote verification process that functioned something like this.