Why did Tokugawa Ieyasu, the first Japanese shogunate, reign for such a short time?

Mainly to enable their Tokugawa family to inherit the position of shogun and pass it on to their son in advance. First of all, after the Battle of Guanyuan, Tokugawa Ieyasu established a shogunate and became a shogunate general. After this series of events, he is already sixty or seventy years old. At that time, from the medical level, he was already a very old man. In order to avoid being too old to manage the overburdened shogunate, Tokugawa Ieyasu succeeded to his son in advance.

Secondly, although Tokugawa Ieyasu still belongs to Toyotomi Hideyoshi's subordinates in name after the official garden war, if he can't draw up the inheritance rules of the shogunate before his death, the Toyotomi family will probably overthrow the regime he worked so hard to establish after the shogunate's death, which will obliterate all his great achievements.

Therefore, when Tokugawa Ieyasu was still alive, he passed the position of shogunate to the next generation of Tokugawa family, that is, his own son, thus setting the rule that shogunate can only be played by Tokugawa family members. When Tokugawa Ieyasu was still alive, people who were opposed by the Toyotomi family could not say anything. In fact, although Tokugawa Ieyasu abdicated the position of general to his son Hideyoshi Tokugawa after only two years in office, he still served as the actual supreme ruler behind the scenes and trained his son so that he could better manage the shogunate for himself.

It can be seen that people in high positions have made careful plans for the future, and they try their best to let future generations inherit and carry forward their achievements. Regardless of ancient and modern times, Chinese and foreign rulers hope that their political power can be passed down and remain immortal, which is probably a universal feature of feudal society.