There's no reason to assume a priori that an act is immoral. The primary criterion for an act to be immoral tends to be that it makes someone else's life worse.

There is no direct effect of piracy on the author of a work. There is no physical carrier particle for piracy, and no sense organ for "being pirated"; pain receptors do not immediately fire. The indirect effects may include decreased numbers of purchases of copies of that work, unauthorized derivative works being created, people experiencing the work that would not have been able to otherwise, and so on. It is not obvious to me that the net sum of these is negative (or positive, for that matter), so the question becomes: why do I think it's permissible to expose the author of a work to these indirect effects?

We need a moral understanding of the world to answer this question. Traditionally we start with notions like property and autonomy: you can't take someone's stuff, and you can't force someone to do something, or manipulate their body without their consent. However, a piece of data is not property; it's an idea. This is evident when we look at simple data, such as the numbers 0, 1, or 2. They're mathematical concepts. Then we can look at bigger numbers, such as the AACS key, 13,256,278,887,989,457,651,018,865,901,401,704,640. Any cutoff at which we might decide numbers stop being ideas and start being something else (such as property) would be utterly arbitrary, and the goal of our morality is to be just, not arbitrary. Thus, if we want to make it impermissible to pirate files, we have to make it permissibile to censor ideas. Western society generally has come to consensus that censorship is impermissible except, some hold, in cases involving deep physical or emotional harm or suffering, such as child abuse. It seems clear that the indirect effects of piracy do not fall into this category, so these objectors need not be concerned. The real worst-case indirect effect is destitution (starving on the street), and Western morality does not compel one to prevent destitution. You can even look at the notion of "competition" in the market as encouraging one to indirectly work toward the destitution of our competitors, and view piracy as a competitor in the market for copies of the work. Even for those who truly consider it unacceptable to let someone go destitute (who might also argue the immorality of capitalism), it's possible to engage in piracy while working to prevent eventual destitution of creators through other means.

Traditionally in these arguments one then moves the goalposts from moral permissibility to desirability, saying that "although we consider it technically OK to pirate, mustn't we agree that it's not the optimal behavior?".

For one thing, you've already "lost the argument" by moving the goalposts. But if we look at this as an attempt to, rather than pick a winner for an Internet fight, come to a more comprehensive understanding of the world, it is clear why this question immediately comes to mind. People are worried that we'll starve our artists and musicians and authors and programmers, or have to subsist on what little art, literature, and software we can skim off of independently wealthy authors, crowdfunding, and preorders. They see the creative output of the current world and worry we're going to change circumstances in such a way that makes that level and quality of output impossible.

My answer to this concern is that there's a huge amount of creative work being done right now that is illegal, and operates via happily evading enforcement. Even the portion of this that is legal, protected by "fair use" clauses or licensing agreements not converted into machine-readable form, is often deleted by automated systems which cannot process intent; this happens widely on YouTube. There's another large category of creative work that is simply not done out of fear of breaking copyright laws. This is every TV show that ever failed to win licensing rights, and a bajillion pieces of unwritten software; this is unreleased albums rotting in archives because samples couldn't be cleared to release them. It is not justifiable to short these authors and works, who are being censored now, for the sake of those who wield copyright and censorship as silencing weapons. I do not have the hard numbers (and honestly this is not a numerical question) to say the world would be more musical, literarily richer, or have better software without copyright. If this is a question that can be objectively answered, I can't exclude that its answer may be negative. But I do have an argument that says the only ethical option is for us to move toward inhabiting that world.