[Edit: A programmer should be able to Reverse Polish Notate, nested-paren a tree, reduce some arrays, make references in nested scopes, evaluate without mutation, send a message, spin up a fail-fast supervision tree, produce finite answers from infinite graphs, synchronously react, rendezvous with logic variables, combine the above. "Specialization is for insects."]
(But I was pretty serious about that list: I would hope a programmer just out of school would have heard of all of them, and expect anyone experienced to have done most of them, if not necessarily in the languages that make each one easy. Similarly, from Heinlein's original list, I currently lack only practical experience in planning invasions [though I have theoretic hex-board experience], butchering hogs [though I have a friend who has offered to teach me], and dying gallantly [though I have "I am just going outside and may be some time" as an example to follow should the occasion arise])
And I say this as someone whose full time job was to do research prototypes of robotics libraries for a while.
This is my personal opinion, but I think that domain expertise is a much stronger component of bus factor than language choice is. You can write unmaintainable code in any language. But good code often still has prerequisites to understanding. Meanwhile, if you have the domain expertise, you can get quite far in understanding semi-bad code just by pattern matching on manifestations of your domain.