I've been thinking a lot about Terminator 5. Here is an aging Arnold who isn't quite in "tip-taaahp-shape" as he was. Would it be a mistake for the scriptwriters to write in an aged Arnold? Would the "living tissue" over the metal endoskeleton age? (Which suggests that a T-101/800 got boxed somewhere for 20+ years). Would movie viewers even want to see and old arnold?
I think some of the recent negativity towards a CG Arnold is due to how poorly the MacG arnold looked. In Tron: Legacy, Clu looked decent in some shots, but kinda lousy in others. Even the director had said that the age regression didn't look 100%, one hundred percent of the time.
However, it's possible.
First, the McG film's Arnold looked like a painted texture over a 3D model. I didn't get a sense that the skin had any translucence as I did when watching Benjamin Button.
Second, it can't be that hard to find a plaster casting of Arnold from back in the T1 days or from any movie filmed within 5 years of the original Terminator. When you take a casting of an aged actor and try to regress the age in the sculpt, you have to factor in bone and muscle degeneration, and not just sagging skin. It may be that you'd just have to get the most talented sculptor possible, scan the sculpt, and build a 3D model off of that with skin translucence.
Third, there's Roland Kickinger. His form and build made for a very convincing Arnold. In Tron Legacy, Clu was portrayed by a younger actor who provided the body. That actor mimicked Bridges' performance, and Bridges had white dots on his face for motion capture data that gave the 3D model of Clu's head life.
So the way I see it, Arnold would act with dots on his face. Roland would duplicate the performance. Motion capture could still control a 3D younger Arnold head. That Arnold head would be critical to the success of the film.
Yes, it can be done... someone basically has to do it right. It would be funny if the replica prop community did something to show the studios what level of quality is possible if you really put your heart to it.