Lord Vishnu taking an avatar in 21st-century is an absolute truth of spiritual life. The present 21st-century belongs to Hinduism. India (erstwhile Bharatvarsha) will regain its lost glory. India will no longer remain a mute spectator.
Lord Vishnu shall manifest form of Kalki avatar in less than a decade from now. To annihilate dark forces of nature, to re-establish Dharma (righteousness) coming of an avatar in present times is an absolute must. The society cannot sustain any longer of its own.
Terrorism worldwide is baring its fangs. Nothing could be reverted back practically. In every country, corner of the world the situation is getting from bad to worse. Mankind would shortly witness the coming world war 3, a Mahabharata of present times, a man-made calamity.
The start of a Mahabharata announces advent of an avatar as per Hinduism scriptures. A mother of all wars, Mahabharata is a dharmic battle fought to re-establish Dharma. Only then the society can be cleansed of all its ills. We can safely expect 20% population of the world getting wiped out.
It is in such times, in the aftermath of world war 3; humanity requires a self realized soul, an awakened one who can provide a soothing balm to the hurt humanity. A knower of all, a self realized soul mounted with power of Bhagavad Gita and Upanishads (independent spiritual treatises) finally shows to entire mankind the pearls of wisdom contained in Bhagavad Gita.
In absence of teachings of Bhagavad Gita life would practically lose its meaning on mother earth. 1200 million bodies lying all over world would be catastrophic. Still, humanity has to survive, rise from the grave. It is Kalki avatar, a manifestation of Lord Vishnu who shall show to entire mankind the merit of indulging in spirituality to reach state of self realization, god realization.
What about beings from other planets/ star systems who are currently most concerned about ours? Many believe that they will step in to avert disaster. The mess we make here affects them too, besides, they care about us and life here. They are not part of the Hindu belief system? In my experience they have been helping us and interacting with us for a long time.