The problem isn't that it's "considered sub-standard now," it's that, compared to 35mm film, it was sub-standard then, too.
But it doesn't really matter since the vast majority of ROTS was CG anyway - the maximum resolution of that film, even if it were shot on 35mm, would have been limited by the resolution of the CG shots - and I don't think there's a shot in that film that doesn't have at least some CG in it.