Brisker Rav on the dual purpose of the makkos

Rashi explains the pasuk "Re'eh nesaticha Elokim l'Pharoah" (7:1) to mean that Moshe was appointed as a dayan, a judge, over Pharoah. The Brisker Rav explains that the makkos were not just a means to force the Egyptians to free the Jewish people, but were necessary to fulfill the promise to Avraham of "V'gam es hagoy asher ya'avodu dan anochi", the promise to judge the nation who would enslave the Jewish people. Therefore, Moshe had to act as a judge to impose the necessary punishment on Pharoah, on his servants, and on the Egyptian nation, each according to what was deserved.

I do not understand this insight of the Brisker Rav. The implication is that the judgment on Pharoah and Mitzrayim is a "chiddush din" which warranted the makkos even if the Jewish people could be freed in some other way. But what kind of new din is this? We know that there is a principle of schar v'onesh, reward and punishment, which means good people eventually reap their just rewards and evildoers get punished. If the punishment of the Egyptians was warranted under the principle of schar v'onesh, then why do we need a new pasuk of "v'gam es hagoy...dan anochi" to tell us that G-d will judge and punish them? And if the punishment of the Egyptians was not warranted under the normal rules of schar v'onesh, why did they deserve makkos at all? (See Ramban to Braishis 15:14 who explains why the Egyptians deserved to be punished despite the fact that G-d had predestined the Jewish people for enslavement. Perhaps that is the chiddush here, but I am still not satisfied -- why would that necessitate the appointment of a judge as part of the process, a concept not implicitly part of the usual schar v'onesh pattern?)