Regarding making the model, yeah I imagine it would be difficult. But who knows with special effects and 3D printing, maybe it won't be long until that technology becomes accurate enough and accessible.
Regarding your other points, one would of course have to have the model face ready to go, then steal the phone, turn it off so that it can't be remotely wiped, and bring it somewhere with no data reception to do the face scan.
But again this would only ever happen to high value targets like big celebrities. I'm just posing theoreticals. I think it's important to do when security is involved.