If the GPL can legally claim that anything linked with something already GPL'd MUST inherit the GPL regardless of whose work it is because that makes it a derived work, then SCO can make the same claim that anything developed to be linked against something with their copyright/license must inherit that.
You are completely wrong. It would be like claiming that if I took a piece of code I wrote myself (FooShell) and linked it against Readline, creating a GPL-licensed combined work, then FooShell itself would be permanently under the GPL, even if I found a non-GPL Readline alternative later.

SCO's claim is actually even more illogical than that. They claim that derived works become their property. So in the above scenario, FooShell would not only be permanently under the "infectious" license (the GPL), but the FSF would own the copyright on it too, and be able to dictate to me the terms under which I can use it.

Again, the GPL/SCO comparison is utterly bogus. A work combined with a GPL work requires the whole of the combined work to be covered under the GPL, not the individual components, and it certainly doesn't give anyone else authority to dictate how your part of the combined work may be used in other pursuits. That is, however, exactly what SCO is trying to do with respect to IBM's JFS code.