The only unstable factor for Jing Ke to stab the king of Qin is that the state of Qin may compete for the throne because of the sudden collapse of the monarch. For example, the son fights for the position and the general is in power. Only when this kind of civil strife happens can the already powerful state of Qin decline.
Extended data:
When Wang Jian led a 600,000-strong army to levy Chu, he was afraid of the suspicion of the late emperor. Meng Tian holds 300,000 troops, and Fu Su, the eldest son of the first emperor, still takes orders from Hu Hai's butcher's knife.
It was Li Si, an important official who held power after the death of the first emperor, who mastered the central civil service system. Reese himself doesn't have too much political ambition, so he won't interfere in the internal affairs of the Qin royal family, and will only wholeheartedly assist the new king of Qin.
More importantly, before the arrival of Jing Ke, Ying Zheng's brothers were killed by Ying Zheng in a failed coup. And this childe, except Fu Su, the eldest son, is very young. Hu Hai, the future II, is only three years old. At this point, Fu Su was fourteen years old, one year older than Ying Zheng when he became king.