As I said before... SHINICHI AND CONAN SEPARATE. THEY ARE NO LONGER THE SAME PERSON! XD Therefor, Ran gets Shinichi, Ayumi gets Conan... happy ending.
Nobody is saying that they will get into a relationship right now if he stays Conan but eventually Ayumi will grow and be on the same level of intelligence, which would mean he couldn't see her as a kid anymore. He won't have adult experiences that he would get as Shinichi be re-growing up as Conan. He'd just re-live everything and grow no larger in mental capacity. That means he wouldn't see Ayumi as a child anymore. He could look at her as a respectable partner. And it's not clear right now as to what his feelings are for her. He turned down the one chick who was the soccer manager after she confessed... Ayumi has actively kissed him, pushed other girls away from hi, and even said "That's why I love you, Conan-kun!" before and he never said a thing. So his feelings are unclear.
AyuCon is not impossible at all. In fact, I'm willing to be dollars to donuts it'll end with AyuCon AND ShinRan. It's the only way to have a happy ending!