It is true that color films have several layers but each one is very thin. There was no problem in getting high sharpness from Kodachrome 25. Kodachrome 64 was never nearly as good as Kodachrome 25 and that same difference existed between Kodachrome II and Kodachrome-X. I also shot Kodachrome briefly in 120 size while it was available. All maximum resolution numbers are based on high contrast ratios. I agree with a previous poster who pointed out that Ektar 100 is at least as good as any post-Kodachrome 25 slide film. If you can't achieve high sharpness with Ektar 100 it's not because of the film or any test or review of it.
When it comes to b&w negative films, TMX and ACROS are not nearly as fine grained as TP, ImakeLink HQ, CMS 20 or Agfa Copex Pan Rapid. I shoot Ektar 100 in 35mm and 120. You have to make a pretty large print from a 6X7 Ektar 100 negative to see grain or any loss of sharpness. As slide film use declined, all of the R&D for color films went into color negative film. Technology from motion picture film was also transferred to still film formats. That's how we have the high quality Portra and Ektar 100 films. Slide film hasn't been improved for some time. I also agree that basing things too much on scanning is likely to give erroneous results. You're just adding another step with all of the potential loss of image quality that can cause. If I know I will need a large print I would rather just use a larger format.