According to the history of the world government, 20 kings banded together forming the Ancient Alliance to supposedly overthrow the Ancient Kingdom. This Ancient Alliance became the world government and the twenty kings became the founders of the world government. The Tenryuubito descended from them.
Now for these twenty kingdoms, the respective twenty kings had to have been extremely strong to be the leaders of the alliance. The leaders of the pirates the Yonkou are very strong and the leaders of the marines the admirals are also very strong. So the leaders of this twenty kingdom alliance had to have been very strong.
That means that their descendants are also supposed to be extremely strong meaning the Tenryuubito are supposed to be very powerful fighters. However the descendants got complacent with their superiority and became extremely lazy and weak resulting in the pathetic Tenryuubito we have now.
But what if there are a certain group of Tenryuubito that chose to remain strong. They secretly kept on training and fighting thus retaining their power from the original twenty kings. As a result these highly secretive Tenryuubito are pretty much the "true Tenryuubito."
Maybe in the upcoming war, these particular Tenryuubito may be forced out into the open.