(Let’s not discuss rivalrous goods; they are not relevant to the question at hand.)
> What is the negation of the statement below?
I don’t understand what you mean. The text below is a question, not a statement.
Software is not a scarce resource; once it has been created, it can be easily defined as non-scarce. And before it has been created, if there is sufficient need for it, someone will pay to have it created.
> why not use it only for Y years and give it to the public for the common good?
Well, some people think this would be a good idea. Other people call those people “communists” and/or “anarchists”. So, the question is hotly debated, but also, for our purposes, irrelevant. We are debating the property or non-property status of ideas, information and software.
> I know what rivalrous goods are. What on God's earth makes them so special and different from non-rivalrous goods?
[…]
> We decided to [grant them special rights] as the other option was barbarianism and anarchy.
And this is exactly why rivalrous goods are different. We have (most of us) decided that we agree they need to be granted “property” status.
However, I do not think that defining software, ideas or other information as a non-ownable non-property would be “barbarianism and anarchy”. If you think so, please elaborate.