Has metal already been perfected? Can anything as good as the classics still be made?
No, I'm ternally optimistic in that regard, if there was no room for advancement left metal would already be dead from endless recycling instead of just being halfway there.
It can't be hard to be better than silence, can it?
Silence is not music. Music is the best.
Is it a betrayal of Art for a band to consciously appeal to fans it has already attracted?
I'd say it depends on whether the band's goal is to create art and not product in the first place. Consciously appealing to fans with more mainstream tastes is betraying the muse.
Should a band care about its fans?
Absolutely.
Should a band worry about whether people like them?
No, this almost always leads to pandering & uninspired material.
Which is better, to sell your ideals for a good price or to stick to them for long enough to make those ideals as well as yourself seem ridiculous?
The latter, but only because the other option is selling out, when you realize that standing up for your beliefs has led to self-parody you need to seriously evaluate your position.