Why does history define the prosperous Tang Dynasty as the period from the beginning of Tang Gaozong era to the end of Tang Xianzong era, that is, 650-820, the Anshi Rebellion?

The Anshi Rebellion should only be a turning point, not a sign of the end of the prosperous Tang Dynasty. The Anshi Rebellion did not fundamentally shake the rule of the Tang Dynasty, which still had enough wealth, soldiers and popular support in the world. The biggest evil result of Anshi Rebellion was that it brought a buffer region. Because the Tang Dynasty was unable to annihilate the rebels, it had to place the rebel leaders on the spot and acquiesce that the leaders of the buffer regions could be hereditary, which led to continuous wars with the buffer regions in the Tang Dynasty 100 years.

It can only be said that the decline of the Tang Dynasty after the Anshi Rebellion does not mean the end of the prosperous Tang Dynasty.