There's no "natural" reason why the subcontinent should be united or divided, rather the governing states that exist at any point in time are a function of history. Like Europe, India has at times been divided and other times united as a single polity (first under asoka ~300bc, then akbar ~1500ad and finally the Raj ~1850ad until independence).
If by "further" you refer to 1947 division of the subcontinent then it has in fact broken up again, in 1971 when East Pakistan became Bangladesh.
There is great value in being united of course, which is also the original intention behind the EU. When constituent states disagree or want to split up, like Telangana and Andhra Pradesh did recently, they can do it through a constitutional process rather than a war. Being a larger state also provides greater opportunities for its citizens.
As time has passed, the part called "India" today has faced a few separatist movements, but has held together primarily by creating a more successful shared narrative so it means something to be "Indian" in addition to being a Punjabi or Gujurati or Muslim, Hindu or Jain. This is likely to continue.
I was thinking of the contrast that serious separatist movements have gotten no traction since the Civil War in the US, but India has had two of them and wondering if a third was a thought.
Which two are you referring to? The first separatist movement of course was the demand for Pakistan prior to independence. Then there was the demand for Bangladesh in East Pakistan. Both of these were successful.
Within today's India, there was first Tamil separatism, then Sikh, Kashmir and then a few small eastern tribal groups. They have each waxed and waned over the years for different reasons. Ultimately it is the duty of the Indian state to preserve & strengthen the Union while ensuring that its value for each constituent outweighs any ethnic or religious separatist sentiments.
The US is unique in being formed basically from scratch, and growing from there, rather than as a merger of historical nation/states.
I was referring to what you referenced. In a lot of ways the US was just another part of England, but the revolution really started as people wanting their rights as English citizens and went separatists from there. We then got waves to different regions as immigrants (e.g. the North Dakota and Minnesota immigration maps are pretty interesting for an example).
It just seemed like our Civil War shutdown the separatists and India separatists had some success, so I was wondering if there is a likely further break or not.
Friedman had not won the Nobel prize during the time of this writing. Only later did he gain prominence and influence the economic policy making of several countries.
So many decades lost.