I do not think, there is any valid point in Bal Thackerayji's argument. Mumbai or for that matter any city in not built by the locals alone. Rather, the contribution of city building from the migrant population is more than the locals. Why people are migrating from rural areas to urban areas? Because of job and business opportunities. There are many villages in India, which are abundance with various products but those products have neglible value in those villages since there are no takers for it. That was the case with Delhi a couple of decades away. Demands were few and supplies were high. But as the city developes and more and more migrant people came and settled, it took the reverse turn. The locals who were till then hands to mouth, started flourishing. There were takers for their product. TEnants compete with each other by offering more to the landlords (who are all local people) Thus, overnight the locals became rich. It is not only the story of Delhi, but it is the same with any other city. Local people in Mumbai and its suburbs have benefited immensely by the migrant population. Rather the local economy of any city is controlled by the migrant population. If not for migrant population, these places would have remained the same, local people would have remained poor.
Bal Thackray Sahib is one of the proponent of Hindu unity, and it does'nt speak good of him if he prefer to remain as a Maharashitrian than an Indian.