There were some mistakes made in her career I can agree but I think even with the very best career trajectory possible, she was always going to "decline" somewhat just like every single musician ever does as they get older.

She's getting older in an industry where youth is everything, perhaps slightly moreso for women but even for the men too. Even male recording artists, name one who didn't decline once he got into his 30s, 40s, 50s and older. That's just the way of the music industry, you peak in your early 20s when other young people - who are the main ones buying music and determining popular culture - see themselves in you and then you slow down.

Also Mariah had almost unimaginable success in her early career - debuting with 5 consecutive number 1s, continuing to dominate the charts throughout the 1990s, and she influenced the sound of an entire generation of other musicians below her. Not only is it unfair to act like her more modest successes since have been a "total failure" just because she's not still shooting up the charts every time she burps, it's a shame we can't appreciate and just let her rest in her legacy as a musical genius and legend. That's where the real disappointment is, not that her career and mainstream popularity has predictably waned as per what is normal in the industry.