The marginal cost of copies after the first is precisely zero for software.
And the first copy sold doesn't pay for the cost of development - for the current version or any future versions.
You assume that, in the absence of piracy, that some of those people would have purchased, but there's no basis for the assumption beyond the notion that somebody likely would have paid. Maybe somebody would or maybe somebody wouldn't have, but it's completely speculative as we don't know what would have happened.
So because it is inevitably speculative, you think "oh well, piracy is ok because we can't *prove* income was definitely lost"? It's a reasonable assumption that *some* income is lost, even if it is speculative.
And ultimately, another key issue is whether it is right or wrong to appropriate someone's efforts against their wishes.