Actually it would still just be an opinion, as you can't back up saying that certain music is garbage with any sort of evidence to uncontestedly prove it so.
Now, if a few years from now you could provide statistical evidence showing a significantly higher number of people still listening to another kind of music than that, then that would be fact.
well regarding opinions and facts, i guess that goes down to that big philosophical argument that there are no truths and everything is an opinion but if thats true then it leads to a sort of nihilism and theres no point in doing anything because its just an opinion anyways. that was the path smith took in the matrix revolutions and look what happened to him lol.
regarding cookie cutter music, you can judge it by its legacy down the road like you said, or if the bands seem artificial like nsync or DNCE or i forget that new boy band from UK which simon cowell helped make. you can judge a musician by what corporate sponsors they affiliate themselves with like taylor swift made a pretty good album in a sense (of course i dont think to o much of it) and on the other hand she is sponsored by walmart.
im usually quick to judge a musician partly by the lyrical content. so if they are talking about mindless consumerism like flo rida and 50 cent and you could put that against eric b and rakim and you know figure it out for yourself. i still like 50's first 2 albums but i thought the lyrics were pretty mindless.