Shiho doesn't exactly have a life as an adult. Everyone she loved is dead and most of the people who know her want her dead for defecting. As a child she actually has a real life with Agasa, whom she likes as a father figure and doesn't find boring because they have similar interests. Unlike Conan, she actually seems to like school because she never had a school life as a real child. Ai talked about the crushing loneliness of that once, eating alone while being rushed through grade levels and college. I doubt the Black Organization will crumble down neatly. If she returns to being an adult, she will likely have to go into hiding and leave Tokyo, or Japan altogether, to avoid the retribution of the Black Organization's partners. Traitors are hated most of all.
I don't see why people think Shinichi will stop visiting as an adult. He hung out with Agasa pretty frequently before he shrunk. Why would he revert to his arrogance after all the mental growing up as a fake child? Sure Shinichi will go to college/get a job eventually but Ai has plenty of time to work out what she wants.