Immortality never belonged to human beings. It would never be possible for human beings to become immortal now or in future.
Immortality always belonged to the spirit, Prana existing in our heart as our soul atman. This soul atman from times immemorial is immortal, could not be created or destroyed.
In their cosmic sojourn souls atmans gathered impurities and to cleanse self of dross impurities within it plunges itself into endless cycles of birth and death, manifestation after another!
It is only when human beings indulging in spirituality reached state of enlightenment followed by moksha salvation; the cosmic journey of life for soul atman within comes to an end.
Before starting its cosmic journey souls atmans were pure but occurrence of big bang resulted in souls atmans scattering all over cosmos at unimaginable speeds. In the melee hurtling souls atmans gathered impurities.
The physical human form has a limited shelf life of about 100 years as detailed in Hinduism scriptures. Even before achieving age of 100 the individual limbs start failing. Only a very select few individuals crossed hundred years of age.
What use if the human could become immortal? One could not live eternally in a failed body!
Precepts of immortality can be understood in entirety indulging in spirituality detailed in Bhagavad Gita. Unless we believed in existence of God, until we believed in system of God created by him we would never be able to understand what immortality was all about.
Still human beings like Mahavira, Gautama Buddha even Jesus Christ and Prophet Mohammed became immortal not in physical form but through their teachings. All enlightened souls that ever manifest on mother earth became immortal forever. They live through their teachings all the time.
In my pursuit of spirituality I never felt enlightened souls did not exist. If I wanted to know anything said by Mahavira I just indulged in his teachings through contemplation (chintan). It is an absolute fact of life that enlightened ones never die. They all the time existed within us through their immortal teachings.