If that is true, why do I enjoy Madonna or Usher or ABBA (or The Beatles, for that matter), but dislike Panic! at the Disco or Nickelback?
Because those artists speak to you and the latter ones don't.
I feel that there are more objectives at play than most people are willing to admit. People get VERY uneasy when it's suggested that there are objectives when it comes to criticism of art, since they often don't have any objective criteria for like or disliking it.
There is a degree of quasi-objectivity that can come from years of critical analysis by well-informed people, but it's still ultimately subjective because we're dealing with art critique (an inherently subjective field.)
But let's think about it this way: were The Beatles better than Backstreet Boys? The answer is yes, no matter how many people loved Backstreet (and they had some OK songs). Most people would answer yes, but they end up with reasons like "Come on, they were The Beatles! BSB was. . . BSB," which is NOT a reason at all. I can at least explain preferring one over the other.
Are the Beatles better than the Backstreet Boys?
Probably. But that's the best we can say--critical analysis would certainly lead us to believe so, but that doesn't make someone wrong for enjoying the Backstreet Boys more than he enjoys the Beatles. From our quasi-objective critical standpoint, yes, it's generally accepted that the Beatles are much better, but that only goes so far.
P.S. I have to take issue with Nickelback being "one of the better mainstream bands," too. I know you didn't post that, but it still needs to be challenged. Everyone should watch/read/listen to this video and then try to justify that. This song BLOWS. They are NOT a catchy band.
Tell that to their millions of adoring fans. I don't particularly like Madonna either, but I'd be stupid to say she isn't good.
Your "but look at THIS song, it REALLY sucks!" argument is pointless because to people who like Nickelback, the song doesn't suck. We can apply some quasi-objective standards to evaluating the song's quality, but if we do that then it's got a lot going for it because it's composed, arranged and produced in the way that's most conducive to a lot of mainstream rock fans enjoying it, and it has clearly accomplished that goal with flying colors.
Again, why are you holding them up to some goal or standard they never purported to meet? They're not
trying to make deep social commentary or push the boundaries of what their instruments can do; that's not what their music is about. Their music is about entertaining mainstream rock fans, and they do that extremely well. You can't just pick some arbitrary critical standard that the band had no interest in achieving and then say that they're bad for not achieving it.
Suppose I make a smooth jazz album, and that you really dislike smooth jazz in general. You can't start complaining that it's "not good" because it doesn't contain the elements of whatever music
you like, because then you're holding the artist to a different standard than he purported to meet in the creation of the work. If you do that, you're bound to think most music sucks because your criteria for evaluation are so narrow and arbitrary.