Very interesting India actually voted for the PRC's ascension in 1971 ... I am very surprised (guess everyone hates Taiwan lol).
Hmmm ... perhaps India should break the pact then? But honestly I don't think China would ever allow India on the UNSC because India would very likely veto any UN action or statements regarding Kashmir. China allowing India's ascension into the UNSC would be a huge slap in the face to their number one ally and leave them super vulnerable diplomatically. China might have improved relations with India, but this would torpedo its relations with Pakistan. But yeah, I def see why China would refuse Japan's entry.
India has never broken its word. Ever. That will never happen.
UNSC will continue to become irrelevant as India will grow to be the Third largest economy in the world with the Third most powerful armed forces.
UNSC was important in the past when India was poor and weak. Today it does not really matter if India gets that seat. India does not need that seat to deal with the world.
But the world needs India to sit on that seat for UNSC to become relevant again. And that role is only going to get stronger with time.