You can start a distribution with specific features .. Linux has thousands of distros to change the taste and flavor or meet certain features .. all are welcome ..

But if you fork the Kernel .. .. it is suddenly a new system .. splits are very hard to maintain in a large eco system .. eventually, the question boils down to " kill or be killed " .. besides it leaves the entire community torn ..

Fork may be fine where tech is going to change say 90 percent and older project is just not being maintained ..or you are launching a totally new project / protocol . ..