MB+ is 12+ . Next is 5+ . Also I have a Nue Infinity. So, cancelling MB+ and applying core might become hard.
5+ is pretty long history. The score will come back up in 2-3 months.
And cibil says closed card payment history is considered till 10years in case of +ve history and 7 years in case of -ve history. Just that their weightage keeps on decreasing.
So if you aren't planning to get a car /home loan in next 6m, you can close it. You can get Regalia gold/dcb . Difference in rewards will be significant. With wealth gone, you need a new card. Dcb doesn't give rewards on govt spends. Just check if dcb considers it for quarterly milestone or not.