Honestly Khamenei's death is at best a neutral event for US/Israel going forward and there are decent chances it becomes a net negative. In the short run there's a rally around the flag effect and given that this wasn't exactly a surprise the Iranians have already had plans in place, as seen by the rapid counterattacks. In the long run, Khamenei was probably a net negative for Iran with his intransigence on nuclear weapons (just ask North Korea how valuable nukes are) and general conservatism. Sure his successor might be more of the same but the longer the war goes on the more likely it is the hardliners win out.
It does allow Trump to declare victory and just ignore reality like he does when his other policies do not work out. That's probably what Trump really cares about anyway.