I agree that good music is good music, and that popularity isn't always a bad thing. I certainly wouldn't abandon a band I like if they got famous, if they still put out good music.
However, the kind of music that classifies as popular (aka the mainstream) is very rarely something that interests me.
The music itself may not be bad, but sometimes it's just very obvious that the artists have written a song that is meant for radio-play, a song that "clicks" right away, and when you hear it the second time, it offers nothing new to the table.
While some people might like that, it's honestly nothing I prefer, and very often I find it that the songs that click right away for me, are the songs that I start liking less and less with each listen, meanwhile the other songs just keeps on growing.
I also don't like it how much of popular music HAS to follow a certain standard in terms of structure and length, though I like it when artists are able to sorta ignore that, since they know they will sell albums anyway.
In terms of changing sound-wise, I'm all pro.
Even if it's a band where I love their current style, I would much rather see the band try something new, then to do the same album they have already done, in a mildly "new" way, I think in many examples the band isn't able to top their previous album, and in other cases they just become a band that has released 10 albums all sounding the same.