Henry VI of where? Of England? He was also King of France. Granted, he did go insane and was murdered in the Tower of London by Richard of York, but I'm sure there are more useless heirs in history.
King John was pretty rhubarb, he succeeded King Richard the Lionheart, lost most of the English possesions in France, was bullied into giving most of his powers to the Lords in the Magna Carta and then drowned in a muddy puddle with all his jewels.
I think he was pretty damn useless.
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