Understandable, as there is a lot of confusion on this subject. Took me a long time to work out answers I find satisfying.
Firstly, it's a mistake to equate copying with stealing. Copying is not stealing, copying is copying. Vandalism is not stealing. Littering is not stealing, Nor are speeding, trespassing, slander, forgery, creating a parody, taking photos in public spaces, insulting politicians, and a whole bunch of other activities in any way stealing. Nor is buying from Burger King stealing from McDonalds. When you say "stealing others' works" to mean copying, you fall for publisher propaganda that does want the public to accept that copying is in fact stealing, to stir public anger, slandering sharers as "thieves". They're trying to push our buttons.
To really steal someone else's work, you'd have to take credit for it. We have a name for that: plagiarism. You added "selling them as their own" to "stealing others' works", so what did you mean exactly? Were you talking about distributing others' works, or plagiarism? It's a big, important difference.
"they are undeniably causing harm by siphoning money away" Certainly it is harmful to McDonalds for a Burger King restaurant to open up nearby. Now publishers are being challenged by competition that they have never faced before, the Internet. We're 25 years into the rise of the Internet and the Age of Information, and they're still struggling to get it, meanwhile whining how unfair the whole thing is that their old business model that relied on media being a scarce resource doesn't work any more. They want the Internet and computers to be hamstrung so badly they work no better at moving information around than distribution via CD/DVD. That is asking far too much of us all. Just turn the clock back to the 1980s, rip up the Internet, uninvent the CD/DVD burner and hard drives bigger than 40M? They claim that artists will starve, seem to take it as a given that copyright is the glue that makes art possible. They're wrong. There are other ways to earn a living, such as crowdfunding. And it's not a new idea. Classical music was and still is mostly supported by patronage, not copyright.
The harm to us all is huge. Our public libraries should be a lot more digital than they are. Think of it: no more late returns, lost books, waiting for a copy to be returned because they're all checked out at the moment, and most of all, a much, much larger collection and searchability. If you've ever used a card catalog, one with actual little rectangles of cardboard with typing on them, you can see how much computer search surpasses that. Further, our education system relies far too much on private, for-profit printers. We should have a huge choice of excellent, open, online works available and used for basic education, We have some things. Wikipedia completely outstrips print encyclopedias, just no comparison between the two. We should have more, and we would if not for the evil known as extreme copyright,