Putin is holding his power because he set himself up as the final arbiter in conflicts between powerful "clans". This is largely unofficial power, that he holds personally (not due to institutions).
He is preventing anyone else from gaining similar levels of influence. For example, Mishustin's power was cut down by the changes Putin pushed into the Constitution in 2020.
Side note, this situation is 100% typical for autocracies. And once the autocrat dies, the country very often descends into chaos, simply because there are no institutions that are capable of resolving conflicts between groups without violence. Example: Libya.
