I have never felt comfortable with the NNC. Because if nothing else, having required the setting of 4deg nu and 70% N1 - which I agree will keep you safe in the short term - at what point does the NNC - reading it explicitly - allow you to say : OK , now lets fly level for a bit (we are assuming Altitude is 'Reliable' here ) and establish, through the PI Tables, which of the 3 Airspeed Indications we have is 'reliable' - as one of them might be; and if one is, this makes things a lot more manageable. But any deviation from initial memory Pitch Attitude / N1 targets is not mentioned by the NNC. To read it would appear that the initial Pitch /Thrust values are held forever. Clearly this cannot be the intent.
It would be more robust - imho - if the NNC Memory Item was phrased as ' If Pitch Attitude /Thrust for phase of Flight is known ...Set required Pitch Attitude and N1 ; If not known , set 4 deg /70% Clean ( 10 deg / 85% if Flaps extended).
This would cover the Cruise case - last 3 hours at 2.5 deg nu and 85% N1 - as well as the TO case. We know - or should know - that the required Pitch Attitude /Thrust immediately after Takeoff is 15 deg nu, or thereabouts with Take Off Thrust , and the Thrust is already set. (A/T disarmed) . By reducing pitch and thrust , and in the worst case, you will not stall but you could compromise terrain clearance.
Perhaps it would be better to maintain 15 deg nu / TO Thrust ( and config) until you are above MSA , then fly level and find a thrust to give you the target pitch attitude from PI. You are then flying at manoeuvre speed for the flap ( for Flap 5, for instance, you are now at Vref30 +40. ) Cross check Airspeed indications. Again, one might be correct.
This is consistent with the NNC Objective which includes 'establish a reliable source of airspeed' . And the NAV AIR DATA SYS NNC ( which will be displayed if all 3 Airspeed inputs to the ADIRU are different) will even suggest you select an identified reliable source to the off-side of the flight deck through the AIR DATA / ATT ALTN switch