Consider Mozart's Requiem. Do you think there is any real basis for arguing that it is a 'bad' piece of music? In other words, could we say that someone who judges that piece of music to be 'bad' to be lacking in some sort of discerning ability to even judge or understand what is good music and what is bad music?
Yes, there is a wide basis for arguing. It is a bad piece of music to play in a nightclub while people are dancing.
Once again, definition is needed. Maybe not the term 'good' alone, but the term 'good music'.
And now it gets interesting. You can either argue that music has to fit the purpose - so in a club full of people that are currently enjoying R&B Music,
you can not play Wagner, Hendel, Dream Theater or Monk and expect the audience to judge the music as good. It does not fit.
If you continue down that road, you will quickly realize that you've expanded your term 'good music' into 'good music for [...]' and since that is
connected with an audience which itself consists of subjects - you're in the 'Good Music is subjective' ballpark again.
You could also argue that someone who will prefer Justin Beiber to Coltaine has serious lack in the skill of discerning the quality of music and only
qualified people are able to judge - and after having aquired the necessary education and skill there will be some level of objectiveness because people who are qualified to judge music will agree on certain tracks to be good music - even if they themselves do not like it.
But i think the answer is more simple.
The simple fact that there is such a big argument is in itself proof for its subjectiveness and here is why:
Music serves a clear purpose: It is made to be listened to by human beings for different reasons with different desired effects.
Some trainstations play classical music to get rid of homeless people and drug addicts.
Some joggers listen to agressive music because it spurs them on.
Some lovers listen to music to get into the mood.
Some concert goers listen to music to appreciate the masterful execution of a piece of art and some to bang their heads.
Music is played in the background of shops and in elevators.
Music is, however, usually not played where there are no people who might listen to it.
So there is a necessary relationship beween music and listener. And if the Justin Beiber fans detonate the J-Bomb which will kill every non fan and start repopulating the world with Justin Beiber as some kind of deity ... and generations later, someone discovers an old Hifi set with an intact
copy of a classical genius and listens to it - It will be interesting to know what this person will think and if it even will be able to identify this as music since there are no beats and no synths in it.
You can see it in ethics:
What is considered good and what is considered bad behaviour is subject to change over the years.
The same might go for music.