The proof of crazy is in lack of staying power, not related to talent. The late Amy Winehouse was very talented, but also crazy. Ditto Michael Jackson, and Whitney Houston. And that amazing actor guy who died of a drug overdose.
Madonna is not nearly as talented. But she is not crazy and is thus still around. Hopefully Lady Gaga (who is not very talented, in my opinion) will continue in Madonna's spike-heeled footsteps and be still around in 20 years. Because some people do like her.