Islam did not came from outer space propagated by aliens. It is a native religion from our mother Earth.
What is considered native varies person to person. You consider Hinduism, a native religion because you consider entire India is your native homeland. While people in Bangladesh like me consider only those things native which originated in Bengal. In this sense, Hinduism is not a native religion of Bangladesh or even entire Bengal region, because Hinduism was developed in north-west India. Far away from Bengal.
You BJP-RSS bhakts should come our of this delusion that, at some point of future, there will be a large scale conversion(or ghar wapasi as you call) to hinduism by sub-continental Muslims as a prelude of establishing Akhand Bharat. It will never happen. If Islam ever weaken, it will loose it's adherence to atheism and agnosticism, just like Western societies undergone from Christianity. But that to happen, Hindus also need to become agonstic and atheist and stop identifying as Hindu. As long as Hindutva identity politics alive in the Sub-Continent and talk of Akhand Bharat, there is no chance that Muslims in this region will abandon their identity as a Muslim like what happened among the Western Christians. Here religion, identity and nationalism are closely linked, unlike the Western countries.