Since you say there is no scarcity, create Photoshop for me without making a copy. How about a Metallica album? Oh? you can't?...that means that not everybody can create it..and there is scarcity. You (and every other person that tries to make the case for piracy) claim that the scarcity is in the copy..when it's actually in the creation.
"Indeed, even when digital media is distributed without DRM (as is increasingly the case for music through Amazon MP3, iTunes, and other distributors) the world doesn't end, and artists still make money."
You need to look at the big picture. Movies (and anything digital) are only about what people are willing to pay. Right now, piracy still has some stigma attached to it and most people will pay rather than pirate. If everyone knew they could get something for free, the value would eventually be $0, which will make it impossible for anyone to make money (or even recoup the initial costs). Since software isn't worth $0, I would say DRM has done a pretty good job.
A good example of this is iPhone apps. For the most part, you can't charge more than a few dollars, because that's what people are willing to pay.
"There are many examples now of artists giving away their work in exchange for variable donations (down to none), many of these have been hugely successful in generating revenue."
This isn't a viable business model. Donations only work so far. There are also many examples of people that asked for donations, had thousands of downloads, and got very little in revenue.
It's funny there aren't more people that have your views on things such as the GNU/GPL licenses. Since there is no scarcity with digital goods, am I really hurting anyone when I use GNU licensed code in my proprietary app (the original code is still 'free')? But, whenever there is a discussion on copyright infringement, people talk it's not theft yet when the guy from thesis used GNU code and didn't release it under the GNU license, it was called 'theft' by many people here on HN. It's because of this that I can't take the movement seriously.