Microsoft and a WFC killer app
If Java were opensourced, Microsoft were free to create a non-compatible Java (e.g. resurrect Windows Foundation Classes). Non of the currently discussed licenses, not even the GPL, prohibits this. (And if Sun were to create a special license that would prohibit forking and creating non compatible versions, this wouldn't be a real open source license.)
So, what if Microsoft put a swing-less, WFC-resurrected JVM into all Windows installations _and_ included some real killer apps written against this JVM?
Then everybody would use the non-Sun-compatible MS JVM and nobody would bother to download the Sun JVM. With time even Eclipse would be written against this JVM. Imagine enough users complaining that Eclipse wouldn't run with the JVM on their machines.