My grandfather asks, if Yaakov pointed out that his merit is already diminished, how does that help build his case to get Divine help for salvation? If he is relating in terms of din there are no grounds for requesting anymore. However, as the relationship is one of chesed, there is no set cap, and he can ask for more chesed.
Along the same lines, we say in the prayer of Nishmas, "ad hena azarunu rachamecha velo azavunu chasadecha, veal titeshenu Hashem Elokim lanetzcah" [until now your mercy has helped us and your kindness has not left us, and don't leave us Hashem, our G-d forever] for there are no boundaries to what flows from chesed.
Related posts: http://kallahmagazine.blogspot.com/2015/11/leahs-thanksgiving.html