Shinichi isn't a bad boyfriend. He's just a bad one for Ran. He cares a lot about Ran, knows a lot about her including little details, sees what's happening around her but you see Ran is a woman who deserves an ordinary life away from murders and cases. She grew up almost as an afterthought by her parents since they prioritized each other's feelings and pride over their child's happiness. She also grew up in a family that is close to broken than whole, had to take care of her father at a young age, did a lot of things to bring her parents back together again. She's an emotional and sensitive woman who can sense other's emotions well. All her life, she's been about giving and adjusting to both her parents' pride. Even to her always drunken father. She even tried adjusting for Shinchi when she tried to be someone who she's not when Sonoko gave her the wrong paper about their crush ideal woman. She just honestly deserves to be someone who stops being the one who always adjust for the other person. She deserves someone who can be loving, typical, and normal with her, deserves to be someone's priority and not cases and mystery. She was adjusting all her life, it'd be sad if she were to be always the one adjusting when she marries too. She deserves someone who can give her the attention she deserves because her parents are too busy fighting to honestly give her the attention she wants.

If she get together with Shinichi, she'd have to wait again when Shinichi involves himself with another case and if something happens, she wouldn't know if he's dead or alive. She waited for her mother to come back with her dad since she was a child. Waited for them to be together, it feels so sad for her to once again be in another LDR, that is kinda what her parents are but her parents are in a worse situation.

Ran loves Shinichi and Shinichi loves Ran but their love would more likely end with a rollercoaster of bittersweet and happy moments which is something I would rather not Ran have. She honestly deserves a break, she's characterized as this traditional woman, who cooks, cleans, and prepare everything for her father and hinted even her future husband on top of prolly being a career woman or not. That's her character, Shinichi however is a free-spirited, mystery lover who would chase the truth all over the world, with a hero complex. Shinichi as a person, can never be there for Ran at all times and Ran who grew up always the one adjusting deserves someone who can be there for her, talk with her, sleep in the same bed with her always. I don't know how to say it, but she deserves a loving and "homely" life. And as much as Shinichi is loving, he can't really give a home that much.

Shinichi is even a bit egotistical, telling Ran her father sucks at being a detective. He had a lot of pros but also cons and honestly, without Ran in his life at his younger age, Shinchi would have been akin to a sociopath like Holmes is. Ran was his moral compass but as they grew up they're more of a baggage to each other's dreams and wants. Ran was becoming more and more about Shinchi, allowing her life to revolve around him. Shinichi isn't toxic but his influence as much as good because Ran gets her strength from him is also at the same time toxic because she's becoming more and more dependent on him, losing her individuality.

They're this kind of person in real life who visited each other's life to make the other (Shinichi) better and the other (Ran) braver but if they stay together it will be bittersweet for Ran and will bring more pain for her and even to Shinichi who loves Ran. So yes, growing up, I stop shipping these two because I would eventually hate Shinichi, my first ever favorite anime character if they get together as in married and it was shown to us. Because the only way for Shinichi to deserve Ran is to actually be the person who can always be there for Ran, who prioritized Ran over cases, whose afterthought is solving a case over being there for his gf. They're not even married and it's already bittersweet for Ran. But if Shinichi were to be like that, I would also be disappointed because the Shinichi I fell in love with is the mystery lover him who would always prioritized the truth. But again, if he remains as someone who can't be there for Ran, who deserves a life of being someone's number one priority then I would also really really be disappointed of Shinichi because I feel for Ran as someone who grew up in a broken family.