I remember several so-called forks of Linux kernel. Hackndev (Linux for Palm PDAs) is one of the first that come to my mind. Now hackndev is dead long ago. Probably it still contains some drivers/features not present in mainline, but it's dead. Same will come to "ARM fork" kernel.
In the end everything comes to fear of changes. In the end we have a merged x86 arch. We have first commited in m68knommu and then merged it back to main m68k arch. We have survived major PPC rewrite. I hope that in the end
people interested in having ARM in mainline will have what they want.