There's an old story of the boy who cried wolf: he told villagers that a wolf was coming so often that, when it was true, no one believed him. Mercedes have the opposite problem: they have been so dominant for so long that they could rely on Hamilton's genius and a rock-solid car to win race after race, championship after championship, break record after record.
Nothing in Formula One is easy but getting a great start and bolting out of reach, for so long Mercedes' stock in trade, has made it look simple. And they have been complacent.
It seems that they have failed to develop the thing that wins races when there are competitors: they don't know how to build winning strategies.