high capacity seating
Whih only helps, if you are able to sell all those seats. If you fill an A380, it is a very economic machine. Unfortunately you do only reliably fill it on very limited routes.
Now with all those smaller long range aircraft around (not even talking the very small ones just making it to the market), you do not have to buy size to get range, it becomes much more attractive to buy two smaller aircraft instead of one large, it gives you much more flexibility with only minor efficiency losses.
Time will tell, and I might stand corrected. But I am sceptical whether size still matters and big is still beautiful...
planes equipped with winglets able to complete flights while missing a winglet?
Following according procedures. If you lose a winglet just after rotation without adapted speed, you will barely be able to correct the wing drop.
See the crash of N652GD as an example for selecting the wrong speed in a failure case scenario (in this case one engine out, not one winglet missing).
So noticing that your folded wing is not properly locked at rotation with low margings (which today is the norm, due to flex power), may put you in a very undesirable situation.
If you always plan with enough margin, you lose the efficiency you just gained with the additional wingspan that your folding wing allowed...
787 was not taken to failure- which is NOT a requirement anyhow .
This has been under debate for quite some time... Actually if you demonstrate compliance by "calculation supported by test" (which is possible and done), you need to demonstrate that you identified the right failure mode (as mentioned above, for example compression buckling). So somehow you need to take real or representative structure to failure, to demonstrate that not only the load level, but also the mode of failure has been correctly predicted. For the 777X Boeing may take credit for the 787 Wing test.
With composites it is a bit more complex anyway, as you have to consider an "environmental knockdown factor" for the test not done under the worst environmental conditions (hot and wet). So you do expect a higher failure load under test hangar conditions anyway, above nominal ultimate load.
the wing construction methods and materials have not significantly changed.
Actually the materials have changed much more than the construction methods. Which is why some cynics state that today we build aircraft from black aluminum...