You're not legal (airworthy).
Please provide some kind of reference for your opinion.
Any time you are in a configuration not permitted by the MEL/DDPG, I suppose the airplane is technically "not airworthy" (there was an interesting discussion over the past 2 months in AOPA Pilot magazine by the association lawyer on the issue, and there is NO "black & white"...). However, there are a plethora of such configurations that, even though they would not permit takeoff, do not require an immediate or "as soon as possible" landing. Did the BA pilot who flew on 3 engines from LAX to Manchester (because he ran low on fuel and couldn't make LHR) have a 3-engine ferry permit? Was he "not legal"?
This is what our 747 Classic Flight Handbook has to say on the issue:
There are some situations which always require landing at the nearest suitable airport. These situations include, but are not limited to, wheel well fire, cabin smoke or fire which persists, lower cargo compartment fire, main deck fire, dual hydraulic system failure, electrical faults which result in only critical systems remaining, or any other situation determined by the crew to present a significantly adverse effect on safety if the flight is continued. In each case, it is the responsibility of the Captain to assess the situation and execute sound judgment to determine the safest course of action.
Since the FAA approves the FHB, the FAA has also agreed that except in the named circumstances, it is up to the Captain to determine whether an immediate landing must be made.
THAT is why some would continue to destination, depending on the totality of the circumstances.