I totally agree with you. NDTV is a news website. So they have created multiple subdomains targeting food, movies, sports and are providing news related to that niche. But yes, the root domain should be a multipurpose niche so that the micro-niche of the subdomains are justified. Because of this subdomain strategy of NDTV, they are easily getting more than 50 million visitors monthly by combining all of their subdomain visitors.
Having too many content in a single website can be very hard to handle. So its better to divide the content by creating multiple subdomain for those niche. Subdomains are the future and they can also help create a brand value of the root domain.