If you create an artifical country by force combining several independent states into one, it is destined to be divided once the binding force that held it togehter leaves.
No rocket science there.
Are you talking about Bangladesh and Pakistan ?
The modern day India even has several states which were occupied after British left and fighting for independence, so what makes you think you can force artifical "Indian identity" upon them?
You are talking from the point of view of creation of Pakistan. My take on the matter is partition was the right thing to happen for the wrong reasons, considering how the present state of Pakistan has evolved. But let's keep that aside for now.
Let's talk about how large nations keep it together and why do they disintegrate.
There can be multiple reasons for unity, nationalism, mutual benefit, mutual respect, greater common cause, even sometimes a common enemy, common language - most of the World is divided on linguistic basis, common race, or common ideology , as for example Socialism. India has many of these features.
our founding fathers realized that India is going to be a diverse nation, and we need mutual respect and cooperation with each other to succeed. That's why India is secular, that's why India has so many National languages, that's why we have representation from every sections of the society.
India is a modern idea, much advanced than it's time when it was formulated. The idea is dependent upon realization among the leaders of all the states that, we need to stand united and benefit from each other and set our differences aside. You saw that happening after 50 years, in the form of European Union, which is still in a nascent state, while we have achieved a greater degree of success so far as nation building and national unity is concerned.
The creation of Pakistan was on the other hand an artificial idea, which did not have much connection with the ground realities. It was mainly created by Muslim elites, feudal lords, the oligarchs who wanted to avoid being treated same as the rest of the population. They wanted preferential treatment. But they could not get any staying in India. So, they created a new country where they will have their own set of rules and subjects to rule over. That's why Pakistan never had proper land reforms, while it was one of the very first thing to happen in India.
Pakistan was created on the basis the religion can be the basis of nationality. The idea fell flat on it's face when Pakistan had general election for the first time in it's history and the country was divided into two parts.Interestingly enough, the Bengalis were not fighting for the religion, but for their language, more than anything else. That, in itself had undone an artificial coalition less than 25 years after it's creation.
Probably India will grow bigger now, may be India will form economic Unions with Bangladesh and Nepal and Myanmar, because that is the modern idea, the way forward.