I agree with with many of the things that have been said here, especially about the added burden of maintaining data structures in multiple languages. I also see two human behaviors (as opposed to technical hurdles) that muddy the discussion.
The first is language preclusion: when a developer refuses to work in languages other than his/her preferred language. I can understand a developer becoming incredibly proficient in one language. Unfortunately superb understanding of a language is often accompanied by elitism and snobbery. Ted Tso's comment about "forc[ing] all of us to learn Rust" strikes me as a bit elitist, taking credibility away from the technical argument of increased maintenance burdens.
The second is attachment to our work. When you pour your heart and soul into producing something, it is natural to feel proud and protective of that work. Early in my career I would become so protective I impeded progress and damaged my reputation with other developers and management. I eventually learned to let go and lead, rather than mandate how things should be. I have unfortunately seen many cases both in my work and in my Open Source contributions where maintainers become overprotective and impede progress. I'm not suggesting Rust is the right direction the kernel should take. I am merely restating that being over-protective is one of the reasons disruptive innovation often comes from the outside.