First India did not mobilize all troops to Bangladesh front but mostly to Pakistan front. India had it’s strategic interest to do so that is why they engaged into the war indirectly except in the last -3 days when Pakistan launched aerial attack.Who said the war was only 13 days. That 13 day success was 8 months of mobilization in the making, which included training and arming millions of mukti bahini soldiers.
You think it was 'only' 13 days to mobilize one million troops, thousands of tanks, 100s of aircraft and warships including aircraft carriers for a war effort?
'Most of the work was done by mukti bahini fighters'. If India had not put it's massive military machine behind the effort your mukti fighters would have fought for decades as a low level insurgency?
Do not forget that when India moved against Pakistan, the US sent it's 7th fleet. India decided to go ahead despite such pressure. Within one year not only was the entire war finished but India ensured that Bangladesh was recognized by the UN as an independent country. You think your militia forces could have achieved all this so quickly?
US said lots of thing so is China but none did anything militarily. Yes it would take little longer but not decades as you claimed. It would be impossible for Pakistan 1300 miles away to control 75 million hostile population as it would not be economically feasible. Sooner or later Pakistan would have to withdraw. For example even Soviet Union collapsed fighting an unnecessary war for a decade in Afghanistan. Pakistan was no where near to what USSR was at that time.