Rama and Krishna were not mortal. Lord Vishnu took incarnation for a purpose. Once the purpose is over and fulfilled, the incarnation comes to an end. They cannot be treated as mortal.
Before concluding His incarnation, Lord Krishna imparted many profound spiritual truths to Uddhava. Weeping, Uddhava said, "Kali fears to enter this realm as long as Your feet touch the earth; if You too depart, what will become of all living beings?"
Krishna replied to the tearful Uddhava, "I am withdrawing My manifest form, but I am merging My absolute, unmanifest essence into the ocean of the *Bhagavata*—the nectar of divine stories. One can easily attain salvation simply by reciting and listening to it."
Ramadasu, an ardent devotee of Rama, said in a keertan “ takkuvEmi manaku, rAmundokkaDumDuvarakU…..” - as long as Rama is by our side, what we lack.
So Rama and Krishna exist not only in the past but also in the present—in an unmanifest form—to protect His devotees.
That is why, despite the prevalence of so many sins and atrocities, rains still fall, crops continue to flourish, and the vast majority of people are able to secure two meals a day.
Regarding Parasurama, we have to wait to know the further future purpose of his incarnation. Part of the purpose might be over. That's why Parasurama is still there.
Subhamastu