Unless the sysadmins had a big powerful union (or sabotaged the system on their way out the door) their tasks would just be handed off to other computer literate people in the company who would end up becoming de facto sysadmins.
Especially if the help articles are on an internal network.
I'm confused. All your arguments seem to be for why sysadmins _do_ need a union. If they're irreplaceable (at the current wage level) and underpaid like you seem to claim then it sounds like they're leaving money on the table by not bargaining collectively.