Well the obvious answer is simple. Administrative scaffolding tied to robust military structure. The same formula used by British to chain together a patchwork of varied peoples and then rule for next 200 years.
Now think about this. If a foreign, Christian European strangers could keep the entire sub-continent, that included present day Myanmar [Burma], Bangla, India and Pakistan united for 200 years using no more then 20,000 British officials/soldiers why does is it a surprise you that India is united in it's present form for 70 years? It's no big deal.
As to what I think the future holds I will reserve for now and let people mull over what I said.