They're a monopoly.

We already have a thriving marketplace of seating- it's called the airline industry. You can buy a seat on a plane from dozens if not hundreds of sellers online.

The hundreds of "travel agents" selling through GDS online have functionally no competitive pressure, they're all booking through the airline's CRS on your behalf in the end -- it's really no different than ticketmaster's reseller marketplace.

Airline price competition comes from multiple airlines running the same route.

Yeah, sure, if you don't care what route you fly that's true. Otherwise you may not really have options.