I speculate that a good likeness made by hand will always be favoured over a scan of an actor's head. Some companies are already scanning heads and it's not incredible. I don't think an exact copy of a head is actually the key to a pleasing head sculpt. Probably a combination of the two would make more sense, a copy of the head that's then built upon.
Plus, Dutch in Predator, Alfred Hitchcock, etc, all these thins long gone, can't be scanned (or maybe with technology they one day can). On top of that, the trouble with getting actors to simply sign off on figures, to now arrange for their heads to be scanned and to get the picture/attitude/look the toymaker wants, I don't think it's ever going to be some easy/perfect solution that can replace sculpting or will be preferable to looking at photos and sculpting.
Where I see such things going, anyway, is maybe a future where people scan their Hot Toys head and put it up on a site and we can download it and make easy bootlegs. Things like PVC figures you see out there, with their simple, solid shapes without moving parts, could be scanned and illegally shared and printed at home. That to me would be an exciting future.