An avatar in Hinduism incarnates when Dharma (righteousness), not Hinduism takes a downturn. As per Hinduism scriptures life comes full circle every 3500 to 5000 years. At the end of the era manifests, incarnates an avatar, a messiah savior of that era.
The need for an avatar to manifest arises as re-establishment of Dharma could not be accomplished by an enlightened one, a man God alone. When the entire humanity is facing a downturn, even self realized souls like Mahavira, Gautama Buddha and Jesus Christ and Prophet Mohammed could not do much in the matter.
An avatar is a deadly combination of a self realized soul coupled with powers of Chanakya. To eliminate misery of millions of countrymen and soldiers Chanakya got 100 kings eliminated in one night. As per Lord Krishna death of few for the sake of Dharma was not termed a sin. The battle of Mahabharata was fought to re-establish Dharma (righteousness). It was never a normal war.
An avatar is needed in every Mahabharata. After millions succumbed to death world over life practically loses its meaning. To give meaning to life jnana wisdom contained in Bhagavad Gita is necessitated. Mounted with administrative capability of Chanakya along with jnana wisdom of Bhagavad Gita an avatar like Kalki would finally succeed in demolishing dark forces of Kali Yuga.
The status of an avatar is such that one does not belong to Hinduism alone. Mounted with power of Bhagavad Gita, one becomes the guiding pillar of entire humanity. In the aftermath of Mahabharata, the coming world war 3 mankind would look forward to advent of Kalki Avatar. It is only Kalki avatar who can usher the present Kali Yuga to Satyuga, the awaited golden period.
Furthermore, it is not only Hinduism or India alone that is affected by a downturn. The whole world at present is in doldrums. Terrorism is baring its fangs all over. The ruthlessness displayed by China is evident world over. To counter all advent of Kalki avatar is an absolute must.