For some definition of "valid". Nobody is stopping you from trusting a different "parallel universe" branch of the Blockchain, you just don't do it because mos

For some definition of "valid". Nobody is stopping you from trusting a different "parallel universe" branch of the Blockchain, you just don't do it because most likely no one else will trust your work and it will be pointless work. Or you do in fact do it because it's a coordinated soft fork (there is a subpopulation that you care about that will together accept this parallel history and reject the mainline history). Construct an appropriate analogy for the bgp use case.