Well, the image is composed of grain, so the detail is made up of grain. But obviously an image with no grain will be clearer than an image with grain. The digital equivalent is noise, but on high end cameras, and Hobbit in particular, there was no noise.

I would venture to say you just saw a lousy screening. I'll grant you that I went to one of only 4 or 5 theaters in the country showing the film in such high quality (IMAX+3D+HFR) but softening around the edges is due to the projector or print. Especially if it was 35mm that is common, but it could be a poor lens on the projector as well.

Anyway, I've seen a bunch of 70mm, and not old, faded beat up prints, but 70mm in no way at all rivals, let alone surpasses modern IMAX, especially from high-res (ie 4K+) HD, digital projections, assuming the equipment is in good order. Just speaking in terms of science, it's literally not possible when you have such pristine looking films like Prometheus and Hobbit. It doesn't help that most 70mm are blow-ups, with an extra three optical stages on top of the 35mm versions. I guess if you could project the negative it would be different, but almost no one has seen what a negative projection looks like (I have, but only for test footage).