This is caused by the natural laws of history, and the replacement of dynasties reflects the will of the people. Although the Southern Song Dynasty was very developed, the development of various industries was uneven. Song Dynasty is a dynasty that worships literature and martial arts. In terms of official system, there are too many people who do not seek political affairs in office, which leads to serious tax burden of the dynasty. All this should be blamed on farmers. Therefore, people's hearts are completely lost. So extinction is a matter of time. After all, the contradictions accumulated in the Song Dynasty over the years have reached the extreme.
Moreover, neighboring countries, because of recuperation, are stronger than Ma Zhuang and are good at fighting. They can't be unarmed against the civil servants of the Song Dynasty, but there are very few generals in the Southern Song Dynasty. Because they can't be reused, they naturally don't work hard. Yue Fei is also difficult to sing, so it is only a matter of time before they perish.
Although Yue Fei was very strong, he could not resist the trend of progress, so the Southern Song Dynasty perished.