^^^That's true to an extent.
However, there are two main forms of music; Art music and Folk music.
Folk music relies almost entirely on "feeling", and one of the whole points of it is that anyone can play or write it. The only way to measure the quality of folk music is through the subjective ear - although it is possible to apply principles from Art Music.
Art music is a different ball game - it's rooted in folk music, of course, but is crafted, with attention to detail and skill. The techinques can be identified and quantified, and the result can be used to make an objective measurement of how good the music or musician is - does the musician fumble whilst playing scales? Does the musician play scales at an impressive speed, and so on.
Just as Folk music can be measured using Art music criteria, Art music can be appreciated entirely from a Folk music perspective - that's one of the great things about music - you don't have to know anything to appreciate it.
As to whether music is crap or not, the most convincing argument is that no music can be crap. If someone created it, then almost always, there will be someone else that likes it.
You can apply measuring criteria to it if you like - and when discussing music, such criteria are very helpful, as they help to provide a path into the music for someone who might not have appreciated it before.
If you just say "Genesis are great", then that's not really helpful to someone who doesn't think they are. If you're describing an album, and you just say "I like it", then that's not helpful - why do you like it? If you say "Peter Gabriel is a great singer", similarly, why is he a great singer? Why not, if that's your take?
The whole "X is crap, Y is fab" line of contradictions and bickering belongs on pop music forums, IMO. On ProgArchives, we have a whole new level of bickering