The flight duration is known, the last ACARS message gives the fuel available then and the arc calculations give an indication of where the plane wound up.
The various climbs, doglegs and descents postulated after the last ACARS message are needed primarily to explain why the aircraft was not picked up by the various radars purportedly scanning the area. If there was not enough fuel to fly this route, then clearly there was a surveillance failure which people are reluctant to confirm.