With respect to the “to have a simple and very reliable mechanism, like a landing gear door.” statement,
this yesterdays (non)event gives you an idea of what I am talking about. And you have to save 1% of fuel on quite a number of flight to compensate for the cost of such (non)events, including a cancelled flight with 100% of unsatisfied customers and 60% of dumped fuel.
For sure 1% fuel burn is a number for long range aircraft, but a drop of 0.1% in dispatch reliability is as well!
Folding wings are definitively something possible, but they come at the price of a few more reasons why a flight might be delayed or cancelled, they come at the price of additional maintenance cost and additional weight. Time will tell whether they pay off in the long term. I seriously doubt, that they are the most favourable design feature of the 777X.