I am the AUDIENCE.

If you hang around on music forums for very long, you will eventually see an exchange like the following:

Poster A: Performer X is all washed up. His new album sucks; he hasn't done anything noteworthy since 1969; [etc. etc.].  

Poster B: Oh yeah? He wrote [big old hit song]. How's about you post some of your songs so we can see if you're better than him? Let's hear some recordings of your guitar playing. Thousands of people go to his shows. How many people came to your last gig? [Grrr grrr.]

Now, it's pretty clear that Poster B above is just butt-hurt because Poster A has, as he sees it, dissed an artist he likes. But rather than just responding with "You're a cretin," and letting it go at that (mainly because such a statement probably violates terms of many internet forums), he brings the "you're not qualified" argument. I'm sure there is a name for the logical fallacy here, but it's common as dirt, and I rarely see a pithy response to it. The important thing to keep in mind if you find yourself involved in an argument like this is that it confuses the roles of the parties involved. Here's how I would respond to that:

I am the audience. That means that somebody (or -bodies) out there want me to give them my money in exchange for watching them play/sing or to get a copy of their record. And that means that I am supremely qualified to judge whether they are doing anything worthwhile or not.

Now, if I play in a cover band (which I do), actually I am probably making that determination with more knowledge and a higher level of sophistication than the great unwashed, but, that has nothing to do with my qualification to make it, because that comes from me being the AUDIENCE. It doesn't matter if they are better than me, because they're not getting paid to be better than me---they're getting paid to be entertaining. And as far as I'm concerned, I can judge that better than anybody. Humans being humans and the internet being the internet, at some point I may well share that judgment with the rest of the world. You can disagree with that judgment, but not my qualification to make it. (Well, not logically, anyway.) Ultimately the music marketplace is very democratic: people vote with their dollars, but even if I am outvoted (and I often am) that doesn't mean I don't get the same vote as everybody else.