Yeah, but who is saying that? Nobody here, certainly. And just because they don't put Michonne with Rick doesn't mean that TPTB for the show are saying that, either. Believe me, I know that there is a history with Black actors being the buddy or the friend or the sidekick, with darker skinned Black actors suffering that fate especially. It's awful, painful and wrong, and still going on today in Hollywood far more than most people would like to admit.
I just don't see that as how things are with Michonne. She's portrayed as one of the top characters on the show, one of the most awesome. She's written in a way that makes her look good, appear beautiful, strong and complicated in her emotions and actions. She's got fighting skills that top pretty much everyone, she's compassionate, she's intelligent, she's got a quirky sense of humor. She's got way more than one dimension, and Danai takes it to another level beyond the writing even with her talent. I don't think that not being paired with the leading man means she's not or can't be a leading lady. She is a leading lady. (Lori was paired with Rick, but I don't think she was ever a leading lady.) That's why I just really don't think that there's an unspoken "we can't have her touch a white man" going on here.