Now I don't think it can't happen like that, but let me provide my perspective. You're suggesting that since in cannon Knockturn Alley is in proximity to Diagon Alley, then it is definitive that one cannot exist without the other. Mind you, I would like this also. However, you have to think of logical space and attraction limits. And in the event Diagon Alley is in fact made into an actual "street", it's doubtful that such a noninviting place such as Knockturn Alley would even be considered by UC.
Not to mention the fact that Universal has really made in no way-shape-or-form a TRUE GEOGRAPHICAL REPRESENTATION of the already existing locales. But it's only because they couldn't, obviously. Hogsmeade is steps away from Hogwarts, the Dragon (arena?), and Hagrid's Hut. Universal took many liberties in their execution of Phase I because it was feasible, both financially and space wise.
But in the event of Diagon Alley becoming an attraction, then yes, I could imagine a few glimpses of Knockturn Alley.