There is no doubt that India will be profitable for Amazon. They're frozen out of China because of protectionist policies, they already own the US and Europe, so if they want to grow, India has to be part of that strategy. Given that their biggest competitor in India is on its heels, I would expect them to throw more money into the country, not less.
It's not that there's absolute certainty that investing in India will be profitable -- it's just that Amazon doesn't really have any better options if they want to continue to grow.
And amazon can not keep spending money on India. They have to convince their shareholders that investing in India will be profitable. So if flipkart being market leader will not be able to convince someone for more money, it will put pressure on amazon also for the same.