It would have been relatively easy to stab or shoot Hitler, but it would not have prevented Goebbels or Goering from taking power immediately afterward without a proper coup.

Hell, this very problem is part of why the Valkyrie bomb plot failed. They had numerous instances to kill Hitler, but all of them would have left Himmler in power and still alive, so they kept calling it off. So they were left with their only shot at all 3 of the people they needed dead being a case where their carefully chosen bomb was not powerful enough to kill everyone, so people survived. Even if it left Hitler paranoid and even more of a junkie than he had been BEFORE the bombing (since now he was addicted to pain meds on top of everything else).