1. Leaving aside the jurisprudential issue of whether there is a general obligation to obey the law regardless of its moral character, whether or not file-sharing is ethically acceptable is not nearly so simple a question as it first appears.
2. "copyright" in the modern sense is an early modern idea, first appearing in 1709, with the stated purpose (amongst others) of safeguarding the interests of authors against printers. The idea behind the copyright law was that it created an economic incentive and reward for creating copyright material, by giving the author a monopoly in the reproduction of that material. The duration of the monopoly was limited (originally to only a few years). Copyright is not therefore something that naturally arises, but instead, a state granted monopoly, created for a purpose.
3. infringement of copyright is unlike theft in that the copyright owner still has the copyright work - they can use it, sell it, reproduce it, and so forth. All they loose by infringment is some potential economic gain *from the person infringing copyright*. If the infringer would never have purchased the copyright work at the going rate, what has the copyright owner lost? The position of people making availible copyright works on a large scale might be different - particularly if they are making a profit, since then there is some definate economic loss.
4. It should be borne in mind that copyright law is far from perfect. Take, for example, the phenomenum of "abandonware" - software (usually computer games) published in the 1980s and 1990s, in which copyright subsists, but which is no longer commercially availible. I can see no violation of ethics in infringment on copyrights which are no longer commercially exploited - the copyright owners, if they attempted to supress free distribution of such "abandonware" would be in the position of the dog in the manger - denying use of the material to others when they themselves are not using it.
5. In the case of emerging artists/ niche buisnesses, the impact of copyright infringment is unlikely to be very great because:
(1) the wide circulation of one of their works is only likely to increase interest in anothers, whether circulation takes place by infringement or not - some artists have reached a wider audience by distributing material for free
(2) if consumers actually *like* a particular artist (particular new or independant ones) they will be less inclined to infringe (and are in any case likely to be able to purchase works at a reasonable price).
6. Taking a wider view, copyright infringment is a symptom of problems with the market in copyright materials - when the monopoly has become some powerful that the copyright owners (often companies, not artists) charge so high a price at the public at large violate the monopoly. The copyright industry itself engages in various anti-competative activities ( proprietary file formats, restricting the use of third party software, the region-encoding of discs) which extend the monopoly in ways set out in law.
7. DRM has not proven terribly successful. Consider, for example, the infamous sony DRM which created security weakeness in computer systems and wrecked disc drives. There is a constant war between the authors of DRM software and hackers, so that money spent on the latest DRM one year is money wasted on a circumvented and annoying peice of software the next. Some buisnesses are perfectly able to turn a profit without using DRM (consider gog.com), and the cost to a small artist of DRM is unlikely to cover any loss that would be incurred by file sharing, even assuming it worked.
8. We are, as you put it "deluged with mass appeal rubbish" because it makes money for publishers. It happened before file sharing and would continue to happen if there were no file sharing. Indeed it is "mass appeal rubbish" most commonly distributed through file sharing.
9. In conclusion, whether file sharing is ethical or not depends on the circumstances - availibility of the work shared, whether the provider or the user, whether the work is in fact commercially exploited etc etc. It is not possible to generalise from the particular situation you have in mind - wholesale sharing of a new, independant artist's work to every other situation in which copyright is infringed by file sharing. Whether file sharing is some sort of act of class warfare is irrelevant to whether or not it is ethical; the common man is no one special.