Britain's demise was relatively swift, and took place over the course of the two world wars. It fell almost immediately into vassalage, under the US. Not quite a bang, but not as drawn out as you suggest.

Its former colonies experienced all I described above and more. In this case, the colonies are most of the world: where are the bases? Everywhere.

With the States, here's the scenario, not too far fetched. We will see 1) constitutional breakdown, as Trump (or his crew) digs in, and 2) economic breakdown, 2008 but exponentially worse.

This would constitute a Soviet scale collapse, to my mind.