Internal IT people would be able to "cheat" by talking to actual users and stakeholders, doing stealth requirements gathering, examining the current system and

Internal IT people would be able to "cheat" by talking to actual users and stakeholders, doing stealth requirements gathering, examining the current system and how it's used, doing rogue user testing, etc. Basically doing all the things that everyone agrees are required for success on this kind of project, but which in the real world end up being dropped or neutered by bureaucracy. E.g., your "expert user" is the manager of the users and doesn't know anything about how they do their jobs, and you aren't allowed to talk to the actual users. An internal person would be more likely to have the personal contacts and trust required to work around crap like that.