There might be comparable forks to Linux out there right now. Does anyone switch? No. Linux is the windows of Unices. You can't dethrone it just by coming up with a better fork.
There's a difference between being slightly better than something that's still satisfactory, and being good in comparison to something that's no longer satisfactory. Look at X11: everyone used XFree86, even though forks were possible—until they decided to go in a direction people didn't like. Then people started using X.Org pretty damned fast.
That was something of a palace coup, though--- x.org wasn't really an upstart fork, but a secession of a large portion of the XFree86 core team. They also managed to get the X Consortium to hand over the old x.org domain to them and bless their version as the new official reference implementation, which cemented their new status.
While I agree with what you are saying, your analogy is not completely accurate. Switching GUI is nowhere near switching the kernel in terms of risk and the hassle that might come with the change. The main reasons for inertia are the linux servers and the development stack that's built on top of linux servers. Most deployed Linuces have neither X.Org nor XFree86 installed on them.
The kernel is already evolving constantly, and a major version upgrade has to be thoroughly tested before rolling it out on a production system. A fork won't be that different - if Linus switches the license tomorrow, most kernel devs will just move to the forked codebase.
A license change is a bad example. Nobody can switch the license for Linux. It has always been GPLv2-only, you'd need the approval of all contributors to the kernel to switch.
