The A380 passenger capacity is waaay bigger than 15-20% of a 787. According to wikipedia:

B787-8 (three cabins): 210
B787-9 (three cabins): 250-290

A380-800 capacity (three cabins): 525
A380-800 capacity (two cabins): 644
A380-800 capacity (one cabins): 853

This is true, but misleading. Nobody orders the "one cabin" version, and I'm not even sure about the "two cabin" version. Realistically, the Dreamliner will seat around 250-290, while the A380 around 500.

I should note, and I am guilty of it too, that the 380 and Dreamliner are not direct competitors. The 747 stretch has a higher passenger capacity than the Dreamliner but airlines were having a hard time filling 450 seats on flights - and asked for a smaller, but more efficient plane. That's the market Boeing is trying to serve here.

And I don't think it's right to compare sales of the A380, which has been in production since 2008 with the Boeing product, which is barely starting to roll off the assembly lines, and say "it's over."

The best analogy I can come up with is that Airbus made "a huge bus", and Boeing is making "a big Prius limosince." Time will tell what the market wants.