Erm, but it is GPL that’s being viral. Not copyright, and also not other Open Source licenses - just GPL. That’s why you won’t get a license incompatibility problem without GPL in the mix.
If you modify a public domain work, copyright comes back. If we're using virus analogies then copyright is like a retrovirus. Forever baked into the "DNA" of our legal systems and you can never be rid of it.
I think, like many people, you consider copyright to be like some unquestionable right. But it's not. It was invented about 400 years ago and it's way out of date. We can and should question it. If you can't think about copyright, you won't understand copyleft.
Does that copyright, coming back, have any actual consequences?
What people mean by virality is not some copyright coming back; it's the fact that GPL "spreads" via linking. And that does have consequences: it's where license incompatibility comes from, and it's also why you can't use GPL-ed code with code under other (Open Source) licenses.
GPL 'spreads' via linking because copyright spreads via linking. If you write a program and I link to it, we both own the resulting work. Your copyright has 'infected' the work. The license in this case can be anything, even a proprietary one, so it isn't the GPL that's special here.
License incompatibilities are certainly possible with proprietary licenses, there's nothing special about the GPL (or copyleft) from other copyright licenses that grants it magical incompatibility properties. Someone could license you software only to use in industry A while somebody else forbids you from using their software in that industry. There are also plenty of non-free licenses that require use to be non-commercial that could face a similar problem.
No, copyright doesn't spread via linking. That's why in real world you need GPL to get license incompatibility - no other Open Source licenses have this property.
I had, and I have provided a counterexample: there is no license incompatibility if you avoid GPL. That wouldn’t be true if the “virality” was a property of the copyright itself, would it?
You're wrong and you don't seem to be listening but I'll try one last time. I think you're simply refusing to engage thinking circuits because you've already installed the following in your brainrc: alias GPL=bad.
Licence incompatibility is not in the slightest bit unique to the GPL. It's simply how licences work. Other licences can seem interchangeable because they are very simple and one may stipulate a superset of the requirements of the other, so you can always "upgrade". But you can't "downgrade". You can't mix a BSD-licensed work with a WTFPL work and license the whole thing under WTFPL because you will violate the BSD licence.
Licences are only a thing because of copyright. Every single licence out there only works because of copyright. Different licences have different aims. The aim of GPL is to "disable" copyright. The GPL is antiviral.
The GPL is only as powerful as copyright. GPL relies on copyright to "spread". Copyright is the virus. GPL just uses the virus against itself in a very clever way.
For what it's worth, the maximalist FSF position on linking is very unlikely to hold up in court, especially not after SCOTUS ruled in favor of Google in Google v. Oracle a few months back (which was not mere linking, but actual wholesale copying of header files).
Doesn't stop me from treating GPL software as a radioactive contaminant in practice.
