" ..............................longest moment arm from the a/c CG so worst-case continuous vibration and prolonged exposure to very low temperatures are just a couple of issues that spring to mind. Then what will happen when the aircraft will land, will have been allocated with a gate and the folding mechanism fails so it needs a bigger gate when none is available? .............
If it were up to you, I suppose airliners would have fixed landing gear. Failure to fold is a LOT less concerning than when the retractable gear fails to extend. Just think how long those struts stay tucked up in the wheel wells under continuous vibration and prolonged exposure to very low temperatures. It's a wonder we don't have a dozen belly landings per day. Oh the humanity.
Yes, distance from the wing fold to the A/C cg is a factor, but the further the better from a design point of view. Long distance from the wing fold to the cg, low g limit and a thick airfoil section are three factors which the 777x would have going for it.
Contrast that with something like the FA-18 which has a rather thin wing and a very high g limit (tricky to design). Or, consider several sub-sonic U.S. Navy aircraft where a large percentage of the span folds; i.e. A-3, A-6, S-3 (also a design challenge)
Folding wing technology isn't new and I guarantee you that if and when a 777x is built, the design engineers working on the wing fold mechanism will spend many many hours studying existing Navy aircraft in spite of your advice that they toss experience with previous successful military aircraft in the bin.
Furthermore, a wing fold failure when approaching the gate won't be the end of the world. What do you think airlines do when a wide-body diverts to or makes an emergency landing at an airport not equipped to handle large aircraft? Pop the slides, evacuate the passengers and crew, and then burn the thing on the spot........... hardly.
Folding wing aircraft may or may not be the future of high efficiency airliners. But if it's determined to be a good idea, building the wing fold mechanism won't be a particularly high hurdle.