PANDAVS WON BUT THEN ALSO KALYUG HAPPENED...

It is a law of nature. As per Hindu Mythology, Each cycle of breath of Lord Brahma (one inhalation and one exhalation) is a life cycle. During this cycle, the living beings should follow 4 yugas, namely

1. Satya Yuga (In short, a Yuga of 100% Truth)
2. Treta Yuga (A yuga with 75% truth and 25% evil)
3. Dwapara Yuga (A yuga with 50% truth and 50% evil)
4. Kali Yuga (A yuga with 25% truth and 75% evil)

At the end of Kali Yuga, it will be 100% evil and that is when Kali Yuga will perish. After this, Satya yuga will begin again and this cycle continues perpetually.

Dharma was won in Mahabharat to eradicate the evils prevailing in the society. Had it not been won, the world would be enveloped into a darker evil even more quickly. and people will suffer even more. So, Dharma was actually

won rather to soften the blow of the injustice that is about to take form in Kali Yuga.

Regardless of Dharma being won, Injustice will take it's place in Kali Yuga. Ramayana took place in Treta Yuga to give moral guidance of what kind of things might happen to the people in Dwapara Yuga so that they can guard themselves. Mahabharata took place in Dwapara Yuga to serve the same purpose for the people in Kali Yuga.

In-short Dharma was won to create a ray of hope that will impend the over-ruling darkness and suppress it in kali yuga.By this logic,when the last traces of godness is lost,kali yuga will become 100% evil.