Excuse me for being direct, but do you work for a record label?
Bandcamp takes 15%/10% (depending on your yearly sales). Combined with the payment processing fees, that gives a return of around 80%, as mentioned in another reply to your post. I think that's perfectly fair, considering the hosting costs for thousands and thousands of hours of music, and their entire platform.
What Bandcamp offers is a sales channel that is already configured, all you have to to is give it a theme, upload your music in a lossless format, name the songs and choose a price. Bandcamp handles the layouts, the tagging, the multiple formats, the embedded album art and the whole shopping experience. You do the art, they do the technical stuff.
You're spinning a yarn that the traditional labels are necessary in order to do marketing, but with the rise of social media and the huge trust people have in word of mouth, the big in-your-face advertising campaigns of yesteryear are simply outdated. The only reason the big labels still have some kind of clout is that they own all the big radio stations. But the rise of streaming and internet radio has eroded that power.
And even if what you say was true, it only holds up for mainstream pop artists, not the much more interesting indie artists out there.