> we still would be paid for the same labour even if someone...illegally have access to the software we build.

Where do you think that money comes from? It comes from the licensing of the software. If everyone is pirating the software, there’s no market for it, nobody’s going to buy it, and there will be no money to pay for your salary.

> that does not mean that it would be significantly affected financially (other than potentially in stock price, which is an entirely different social aspect).

Stock prices aren’t a “social aspect.” They are a financial instrument that reflects the expected future earnings of the company. Companies aren’t going to form and employ people if they can’t sell their stock because their product has no value in the marketplace.

> Windows source code leaked 10 years ago or so. Did it slow down Windows?

You’re asking the wrong question. That leak, in and of itself, didn’t impact the market for the software. Nevertheless, massive piracy of any software would harm the economy. The fact that most people respect others’ labor is what keeps the market functioning.

> authors (owners of copyright) aren't negatively affected if someone creates a copy they would never have paid for.

We don’t know who never would have paid for a copy of software at any price. And there is a difference between knowing that infringement exists and making excuses for it. The question isn’t whether some people do it and the market is still healthy; it’s whether or not we should condone it so that nobody should feel compelled to follow the law. Because, following your logic, nobody should pay for software. If that happens, tremendous economic harm will follow.