We would be guessing what might have been.
What we know for sure is that the region fragmented back into ethnic/regional kingdoms as soon as each empire collapsed.
Isnt that exactly how British left India?
It is exactly the repeat of what happened after the ancient empires collapsed.
As the British left, India was divided into
not dozens, but Hundreds of political units. These units if left alone would have resorted to the same ways of fighting with each other again and again and invited foreigners to fight domestic battles.
Relevant - did Pakistan - one of the fragments of ancient India also not start hosting foreign militaries - US and take their weapons to fight modern India - another one of the fragment of ancient India?
It is the credit of modern Indians then - and Sardar Patel in particular - because of only 3 remained from the hundreds- India, Pakistan and Bangladesh.
Indians invited, cajoled, forced, threatened the rest to join up. It is not the credit of Britishers that India is today as united as can be, but to Indians who had it in their hearts and executed it from the remains of the British empire.
The British left a legacy of a physical and civil infrastructure that bound the region into a unified entity. It is true that Indian leaders post-independence did a good job to continue the British legacy, but the fact of the initial gift is indisputable.
Physical and civil infrastructure is always according to the times.
The ancient empires left their own physical and civil infrastructures. These got destroyed or replaced over time. Then the invasions destroyed the others.
The British did far less for the country than the ancient kingdoms did. That it did not get destroyed is
because Indians joined up and became strong.
The initial 'gift' is entirely disputable.