I’m not trying to cast doubt on your expertise here, and I’ll agree that it’s important to scan 35mm at 4k+ for full detail retrieval due to anti-aliasing and the like, but if DP Steve Yedlin can’t get fresh 6k scanned 35mm filmstock to resolve more than a 3k Alexa in a controlled environment, there is no way a release print suffering several generations of degradation is going to outdo that (especially when created via older processes).
Of course these are generalities and the resolving capability of film isn’t a very linear process (it isn’t with digital cameras either).
This is also a good read as well: http://www.motionfx.gr/files/35mm_resolution_english.pdf
EDIT Yes I can see what you mean about the ribbing. That being said, it’s important to remember that just because you need to scan at high resolutions for details to become visible doesn’t mean those details “exist” at that resolution/couldn’t be adequately resolved at a lower resolution (the reason why probably has to do with Nyquist–Shannon).