As an electronics and software engineer who has read the AD and related materials on the 248 day bug my understanding is that:
The specific 248-day integer overflow was patched, and before the fix was rolled out, the AD required this system to by power cycled every 120 days to prevent overflow
The PCU software still has the
functional requirement
to be able to command all AC GCUs to enter failsafe mode, this means that while the initial bug was fixed, the ability for this particular software system to command the same result is still a functional part of the architecture - presumably for safety management of the AC system
This was not the first or last "software overflow error" issue in Boeing or even in the 787
Although I'm not qualified in aviation engineering I do believe from an engineering safety standpoint that this architecture creates a rare but entirely feasible scenario in which the aircraft would be without AC power for at least 30 seconds until the APU could restore it.
Similar failures have happened on 737s/A320s/A330s and others. I'm not denying it's possible. There's a reason it's a certification requirement for the engines not to be dependent on aircraft power. The APU is MELable and battery starts are not extremely reliable.
I do agree that the engine driven pumps
should
be able to provide fuel alone, the whole point of these pumps is to keep the plane flying within
some
limitations, high altitude is one of those limitations, I propose that there may be others based on the following:
Some more knowledgable people here have proposed or countered vapour lock, fuel contamination and automatic fuel cut-off theories to various degrees - even if these are not enough on their own, loss of electrical during rotation at high temperature could combine with these in a way we have not yet considered
Thrust is nonlinear, and while I'm not qualified to say how much loss of fuel flow or loss of thrust would be critical in this scenario we do know that it was a hot takeoff with significant weight and gear remaining down - I know others here have run sims but I don't think anyone has focused on specific thrust / fuel flow params
While electric fuel pumps might not be physically necessary for takeoff, my final point is: why are they
required
for takeoff? Is it not to mitigate cavitation, fuel sloshing at rotation, or any other kind of problem that might be relevant here?
Thrust is non-linear and complex. Reaction engines (i.e. fans, props) are generally most efficient at minimum power - lowest excess velocity. Turbine engines are generally most efficient at high power. These cancel out somewhere in the middle. With two engines at low power, you also don't have the drag from the dead engine or the drag from the rudder countering yaw.
Cavitating destroys pumps rapidly - someone upthread said replacing the fuel pump immediately is SOP if it has suction fed. Expect end of life in tens of hours rather than tens of thousands.
Some aircraft have switched to using jet/venturi pumps powered by returned fuel, like the A220. The electric boost pumps there are mainly for redundancy and are shut down in cruise; only one in each wing tank. Some A320s replace the centre override pumps with venturi transfer pumps.