Of course if the Magna Carta was such a failure, why are we celebrating it 4/5s of a millennium later?
As we slowly watch our rights being taken by our so called elected officials.
"[1] In the first place we grant to God and confirm by this our present charter for ourselves and our heirs in perpetuity that the English Church is to be free and to have all its rights fully and its liberties entirely."
That said, such documents have a powerful beacon effect. In the case of Magna Carta, it is quite possible that what Jefferson penned 561 years later, would never have taken the form it did--had it happened at all;--had Magna Carta never been forced upon King John.