It gets tiring having these discussions over and over again. Kinsella¹ and others have demonstrated many times the differences between physical property and copyright. It's fine to say that these differences don't apply here for some reason(s), but not even acknowledging them is just spinning wheels, not advancing the debate.
Kinsella's response to pro-IP libertarians is pure hand-waving. He simply asserts that the focus should be on scarcity rather than creation, and proceeds from that premise. His scarcity argument is itself flawed: he focuses on the fact that copies of an intangible good are not scarce, and glosses over the fact the originals definitely are. If you don't already believe his premise his argument is very unconvincing.
He simply asserts that the focus should be on scarcity rather than creation
No, he doesn't. He says the focus should be on scarcity because if it's on creation, and if it's applied in a consistent and non-arbitrary way, life becomes impossible, as "as every conceivable use of property, every single action, would be bound to infringe upon one of the millions of past, accreted IP rights."
He actually has almost six pages explaining the problem with the creation-based IP rights, so saying that he "simply asserts" is just incorrect.
His scarcity argument is itself flawed: he focuses on the fact that copies of an intangible good are not scarce, and glosses over the fact the originals definitely are.
That's because the scarcity of the originals is irrelevant to whether the copies should be considered property, any more than it makes me the owner of the light that comes from my light bulb.
The scarcity of the originals is only relevant vis-a-vis the use of the originals.
¹ See Against Intellectual Property (2008)