Maintaining a fork isn't a big issue if you have enough resources, considering that if Linux was BSD licensed they would still have had their proprietary fork mirror it "close enough" to pull in at least some of the big changes from the main Linux tree.
Android seems to work in this manor to a certain extent (although they open source patches back when they have to because of GPL).
You would most likely have ended up with a bleeding edge open source version and several more conservative closed ports.
Bear in mind that a lot of the reason Linux development is strong is because many of it's developers are paid to work on it and then contribute back (again because of GPL). If a company could gain a competitive advantage by paying the same people to work on a closed fork..
I think the problem with maintaining the fork is the speed of development of the community, and let's face it, it is much easier to monetize hardware than software in the open source world. The fear of being left behind and weird vendor specific errors should not be discounted.
I agree though, someone would try. I just think Linux would have done just as well with a BSD license being released at the time it was. The people and the timing deserve more credit than the license.
You would most likely have ended up with a bleeding edge open source version and several more conservative closed ports.
Bear in mind that a lot of the reason Linux development is strong is because many of it's developers are paid to work on it and then contribute back (again because of GPL). If a company could gain a competitive advantage by paying the same people to work on a closed fork..