because the reality is, some nations have nuclear weapons and colossal economic power, and some don't.
And yet the UK has a veto, and India does not.
Also, only one of the P5 powers had nuclear weapons when the UN Charter was signed.
There definitely needs to be a rebalancing of power within the UN but none of the P5 want their power diluted. And they have the all powerful veto power.
There is no reason we can't replace another government.
The Republic of China did not dissolve.
India is way larger both population and geographically than the UK just like the People's Republic of China is compared to the Republic of China so it seems like there is a precedent to do it.