The easiest way to increase the load capacity of a jet is to increase the wing area. More lifties from the wing, more capacity. But that has drawbacks, such as more lifites also means more induced drag (though wing design can ameliorate this somewhat), and more complexity. There comes a point where wingspan increases not only become more challenging to build, but also impact existing infrastructure.
The parking spots at the jetways at your existing airports are designed with a certain sized aircraft in mind, be it one with the 737 and A320 in mind, or for the 767, 777, and A330.
Boeing is betting that the future of jumbo jets isn’t the A380 sized massive jet (and A380 and 747-8 sales appear to bear this out) but more along the lines of a larger variant of its existing 777 series. But to get the performance they need, they need a bigger, longer wing. But airlines don’t want to foot the bill to totally rebuild airport terminals to accommodate this new, larger wing.
And so, Boeing will add a wing fold mechanism to the 777X, raising the outermost 12 feet of each tip, allowing it to use existing gates that can accommodate legacy 777 aircraft.
Will some pilot somewhere sometime forget to put the wingtips down? I’m sure eventually someone will, though there is a take-off configuration warning system that will (quite loudly) protest if the pilot advances the throttles on the ground without the proper flaps/slats setting, among other parameters.