I’ve heard that at the time of independence only about 40% of Indians spoke Hindi. Why not just try and educate the masses in English, which is also a much more useful language in general?
Hindi is not the national language of India. It is one of the official languages of India. There are 22 Scheduled Official languages and 100s of languages are spoken by the billion people. Wherever you go in India, official documents are in English + one of the 22 languages depending on where you are. Hindi + English in Delhi, Marathi + English in Mumbai, Bengali + English in Kolkata, Tamil + English in Chennai and so on.
What makes you think when 2 people who are native speakers of Hindi or Marathi are gonna speak in English when they meet??? Whoever need English, learn English. Whoever need Hindi, learn Hindi. Otherwise, local languages are very much enough to communicate, find a job, fill forms, find a date, talk with doctors in hospital, lodge a police complaint, understand announcements in train stations, to live a normal life! If a person from Kolkata wants a job in Delhi they'll learn Hindi and if they need a job in USA, they'll learn English and if they need a college admission in Germany, they'll learn German! What is the point of teaching a billion people, a foreign language????