Personally I believe that Shiho is someone that Shinichi can't afford to lose. She is the only person who he can really benefit from in both his social life and also his career as a detective. I feel like if things ever get tough and seriously dangerous, Ran wouldn't be of much help. Shiho on the other hand can handle herself and wont hold him back as oppose to Ran. I believe that after the end of the series, Shinichi would develop as a person and a detective if Shiho was still in the picture.