My understanding of the situation is that the PLT people had some optimizations they could do if lists were immutable, and they noticed that there were very few

My understanding of the situation is that the PLT people had some optimizations they could do if lists were immutable, and they noticed that there were very few places in their code where they mutated lists. So they decided to just make lists immutable and see who complained. And it turns out few other people were actually mutating lists, so they kept the optimization. PLT Scheme still has other mutable data structures that can be used instead of lists, though; I don't know if this is the case for Erlang.