I feel like that if GTAV looked like this

the dissonance wouldn't be there. The same being true for IV.
I think this could work because:
1. GTA's art, as I said above, is iconic. It's still mimicked in pop culture to this day, especially the box art - which is always seen as a huge reveal event.
2. It's one of the few western series' that still releases character art in this manner. It's very Japanese, actually. Japanese developers will often release art for their products and have the games art style reflect that art. In western games, the art rarely reflects the final game release (example: Dragon Age Origins) or they don't release art at all or make whatever marketing materials they do have use in-game models (which is the majority).
3. This allows a sense of balance. The game is no longer overly realistic in terms of visuals, but the character designs are world still are something that mimics real life, so while being stylized it's still instantly recognizable, and given the adult proportions of the characters will give off the tone that it's still a game geared for adults.
4. Due to the iconic art of GTA, I doubt that they will see a dent in sales since everyone worships GTA art to begin with.
And my point was that I doubt GTACW didn't do well because of art style but platform (it did after all start out on the DS).