People say that way of thinking is cynical, but I have worked in the system implementation and system consulting arms of EDS, IBM, and Accenture and that assumption (that the whole thing can be maintained by junior devs one the initial build is complete) is actually how middle managers within client companies have to budget their transition from "build" into "business as usual" stage so that senior managers approve the system implementation/migration.

While the concept that it takes specialized knowledge and experience is true, not having means to retain experience means that it will be chaos some six months down the road when some manager wants to do an enhancement as none of the juniors will understand why certain design choices were made and the implications on the rest of the architecture