It is the same as with Iran's nuclear program, where the goal of Mossad activities is to continually delay their successful attainment of nuclear weapons, rather than totally ceasing their activity. Taking to an extreme the claim that assasinations don't help would be saying that fighting against an invading army is useless because there will be another war a few years later anyway. Assassinations alone will not stop terror but it is one part of a many-pronged effort that includes diplomacy and peace agreements, economic ties, development etc. And if new leaders very quickly take the place of the assassinated ones , then this means the pace and scope of assassinations (and other methods of diverting people from their activities) should perhaps be increased.
Greek PM says may have to call early election (Reuters)
from the article: Mossad killing of terror chiefs has little impact on Israel-Hamas war