I agree with the flaps with respect to complexity, but I disagree that they are good performers. There is an awful lot of wear and tear on the flap hinges and actuation system for practically any aircraft, there is a lot of maintenance involved.
To get the turn back to the A380, its flap system is significantly simpler than that of the 747, still it is more efficient. There are a lot of details where the A380 is well ahead of its competitors of its time. This advantage by now has been deleted with the 787 and with Airbus own A350. Their flap system is even simpler.
The flaps are mandatory, the folding wing is a choice. You cannot avoid the complexity of the flap system, but you can avoid the folding wing. Especially when you have a high capacity aircraft which will only operate from large airports. Probably on many airports you can manage to park 777X between 787s or similar, so that the additional 3 meters do not hurt.
Airbus intentionally accepted the restriction in wing span for the A380, especially after they have learned Boeings lesson that no operator selected the optional folding wing on the 777. It costs some efficiency, but it saves a lot of complexity and extra weight.
A 777 with "permanently folded wings" (aka Winglets) would be only marginally less efficient, mainly depending on the exact mission they will finally operate.
We will see how the 777X outsells the A380. Or not. The time of the very large aircraft may simply be over. Today you can have range without size, you can have higher flexibility with less risk. It might not be a technically (i.e. DOC based) decision to go for smaller aircraft. It looks like the 777X just like the A380 only makes sense for the gulf hubs, high capacity airports far away from anybodies destination, where you need size and range. The 777X will probably never become the Queen of the north Atlantic, maybe an A321 XULR will be instead...