It's more "regression" or something. "Collapse" implies a dramatic event and something completely broken.

The collapse of the Roman Empire took decades and British Empire had a similarly long decline, but I understand your sentiment.

Yes, Gibbon makes for interesting reading. Decline can indeed be prolonged. I suppose it's like how some things get very slowly better over the decades, other things get very slowly worse over the decades.

Centuries, not decades.

I originally wrote centuries, but there’s reasonable criticism calling something a collapse when there’s periods of renewal and expansion in that timeframe.