Once you get into 70mm film or 70mm IMAX, you're better off just leaving it alone and projecting it directly. I think 70mm could be scanned to 8K, and I'm not sure there's even a equivalent for 70mm IMAX yet.
That's why Nolan still screens his films with 70mm IMAX projectors. He might use digital for effects, but I believe he preserves the analog film in the final prints instead of using a digital intermediate.
I don't see any objective reason for choosing 70 mm analog over 4K digital. Sure, the resolution of 70 mm is higher, but does it really make a difference? A lot of digital cinema is "only" 2048×1080 and you usually don't see the pixels at normal viewing distances. On the other hand, when it comes to linearity and color reproduction, analog film doesn't stand a chance against a digital sensor. 2001: A Space Odyssey might look sharp, but the colors are definitely off.
I guess the real reason why Nolan and Tarantino still use 70 mm is not quality, but personal preference and a bit of self-marketing.
That's why Nolan still screens his films with 70mm IMAX projectors. He might use digital for effects, but I believe he preserves the analog film in the final prints instead of using a digital intermediate.