Based on this interpretation of the halacha, with regards to honoring one’s father or grandfather, even after one’s father is not alive the honor of the grandfather should supersede that of the father. This is because we know that the child’s father would still have to honor his father (the child’s grandfather). This indicates that the honor that the child must have for his grandfather is greater than the honor he must show his father. It is not dependent on his father having an actual obligation per se.
Based on this interpretation of the halacha even though Yitzchak Avinu was not alive the kavod of Yaakov Avinu’s grandfather, Avraham Avinu, should have taken precedence.
Perhaps we can suggest that the Maharik knew of this Rashi, however he felt that it was contradicted by the above referenced Gemara in Keddushin that says that the honor of one’s father precedes the honor of his mother. Since the same logic can be applied to honoring one’s grandfather why would Rashi say that the honor of one’s father is greater? If one is obligated to honor his grandfather that honor should take precedence over the honor for one’s father, even after the death of one’s father. Perhaps the Maharik therefore assumed that the halacha was not in accordance with this Midrash. Therefore he concluded that there is no source that one must honor his grandfather.