i agree with the last part...
but I don't understand why people think she can't stay with the professor,AND take the antidote.She could still be happy,act like a big sister for the DB,hanging out with Ran,Sonoko and Shinichi, and taking care of Agasa like Ran is doing with Kogoro.At least she would be more independant than as a child,help financially the professor,being able to defend herself without the need of protection...this is what would be a great ending for Ai.
I remember Ai always reminds Conan that she's not a child,which is true,so what's the point to lie to people around her if there is no danger anymore?