very aircraft has a dynamic power management system that regulates how much auxiliary power is distributed to the overall electricity demand of an aircraft, including the drain from galley equipment to prepare in-flight meals, cabin lighting and those in-flight entertainment screens.
Power supply for passenger electronic devices is managed within those priorities, and distributed in a variable way. There is actually a ‘restricted mode’ in power management for when demand is too high. Power will be cut-off to low-priority (not safety-critical) systems for a brief period.
If every passenger device were connected, it would depend on how that demand affects the supply to other active cabin equipment. The aircraft power management system may switch off supply to the USB charger on a particular seat or row of seats as needed. That power supply would be restored to the seat-back USB device when other demand is lower.
At no point would the power demand from in-seat power outlets risk the safe operation of an aircraft………
An Airbus A330–300 seats approximately 335 passengers (depending on the configuration).
A USB charger usually provides around 1 Amp at 5 Volts. Assuming that all 335 passengers decide to charge their devices at the same time, using the Power formula where Power equals Current multiplied by Voltage (P = I x V) we have P = 335A x 5V = 1675VA. That’s less than the power consumption of an average hair dryer. The amount of power drawn by all 335 passengers would be insignificant to the amount of power produced by the plane’s engines, so nothing would happen.