Ok, not joking now. What Britain and USA are currently doing is giving up power. They have so much internal conflicts that somehow people feel getting on their own will help them. But giving up power rarely made a nation larger and stronger. So they are declining. USA could have been the master of TPP or TTIP, or create some Euro-American superstructure in rememberance of our common roots. They could have been the leader of many things. But instead they just leave and go away. No negocitations. The world will not stop, it will go on, everything else is hallucination. The Chinese are ready to fill the void, and Europe is already bonding with them now that the US don't seem to be interested in anything constructive. So there is no stopping the decline. It is in full speed and it will lead to ugly situations. And as someone who loves(d) the US, this is a pity to me.
Well to be fair some people blame the decline on an increase in minorities and pot smoking hipsters. But I'm generally skeptical of "reasons" because people seem to want always blame everyone else.
Besides, I think the "decline" is a bit overstated. America isn't going anywhere. Things start getting tough and the guns will come out and they have a lot of those. Besides maybe Anglo Saxons shouldn't rule the whole world. And really nothing lasts forever. China might supplant the west as global leader, this was predicted I think two centuries ago, but it won't happen for a while.