The sentence capped Mr. Cosby’s stunning fall from a towering figure in popular culture to an 81-year-old convicted sex offender.
AHD "cap" Def. 4. - "To apply the finishing touch to; complete: cap a meal with dessert.
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AHD "cap" Def. 4. - "To apply the finishing touch to; complete: cap a meal with dessert."
https://www.ahdictionary.com/word/search.html?q=cap
This is a mixed metaphor. You can't cap a fall.