The core is that Linus is quite relaxed about low-problem code, even if it's unmaintained. As a result, there's a decent chunk of the kernel where nobody is doing the work, but not a problem because it's trivial to keep it up to date with core API changes via mechanical changes. There's only one tool Linus has to find people using the code and get them to step up to do the maintenance work - dump it out of the kernel, and wait for a user to appear who can be convinced to maintain the code they care about.