> The travel agent is also not in a position to "unilaterally" fix a bad request, they are also requesting other parties to do things.
Yes, but they're already one level up, so they can fix the problems in their company's immediate system, and then unlike the customer, they're a trusted party in the network of all other parties, so they can mail/call other parties directly and get people there to fix issues without too much delay.
Travel agents were not outlawed. Most people just prefer to save money and do the work themselves (for most trips) rather than pay a travel agent.