1080p is nowhere close to the level of detail a 35mm camera negative has. Hell, I'd go so far as to say that an original camera-negative 35mm frame has at least 8K worth of detail, probably more.
Now release prints, those have somewhere between 2K-4K worth of detail (probably nudging closer to the 2K side).
Even 16mm has more detail than 1080p (though not nearly as dramatic as 35mm).
So yeah, Cameron's full of shit.
So you don't think I'm full of shit, too:
I worked on a grad film last year. The director was contemplating shooting 16mm, 35mm, 1080p/24, or RED 4K. So we did tests for each.
The 16mm and 35mm were scanned in at 4K and 2K (we hadn't decided what to do about the DI at that point, so we did both).
For the 16mm test, you couldn't tell the difference between 2K and 4K. They looked pretty much the same.
For the 35mm, the 4K was the clear winner. You could pick up much more detail than you could in the 2K scan. This was my vote (film on 35mm, do a 4K DI).
Then we compared the 35mm 4K, 2K, the 1080p/24, and the RED 4K.
The 35mm 4K blew them all out of the water. The 35mm 2K looked better than the 1080p, but not as crisp as the RED 4K. The RED footage looked very good, but it didn't have the "feel" of film that I love so much. But as I said, it did look better than the 35mm 2K, as the RED footage was shot at 4K.
While my vote was still 35mm scanned at 4K, the director decided to go with the RED. He did a second comparison between RED 4K and RED 2K. The RED 2K looked a little better than the 1080p, but not as good as either 35mm scan.
Against my protests, the director decided it "looked good enough," and went with the RED 2K, simply because the post-production process would be simpler and cheaper.
What was the point of that rant?
Oh, yeah. There is a point to 4K. It looks better. And even when you downscale it to 1080p, you can tell.
The only thing Cameron has right is that release prints, after they're run for a while, do have less detail than a 1080p digital "print" would, as digital files don't degrade with use like film does. If you release your film digitally, it will always look the same, no matter what. That, in my opinion, is the most convincing argument for digital.