The original Sin City is a stone cold masterpiece and holds up marvelously today. Thing is, it can't be replicated or emulated at this time without seeming like anything more than a cliche because of how many films it influenced sylistically. It's the clear standout as being the original bar-setter, however, which is why its integrity holds strong.
Honestly, I'm not even sure why they thought a sequel, much more one this late, would work at all.