Indian civilization is much older than the hitherto it's considered to be. .Dont think Egyptians are older than us.
Getting back to the topic, how many in those 80% has mastered the grammatical side of the langauge....Even if you make an individual to learn Hindi, once he or she gains some fluency in in the spoken version they would hardly bother to go beyond that..Citing practicality to promote a particular langauage do not provide a solution to any extent or would improve our national integrity, on the wider context the importance of literature to youth would go on a downward spiral - it has already scaled new lows these days.
Already the preponderance of mastering the grammatical side of any Indian language has lost its charm in schools, literature cannot earn your bread and children are more focussed in Maths, Physics, Chemistry that paves their way to professional education and eventually earns more currency. If this trend continues, over a period of time, many of our regional languages would become extinct and adding Hindi to the stack is a lame excuse would only make the current gentrification generation look more silly.
Folks from China or Japan complete their education in their mother tougue but it doesnt take much effort for them clear TOEFL and pursue their higher education in English or settling in the west. Indians are giving over importance to English and the current presque level of conversational skills in their local language be it Hindi, Telugu or Tamil which would count sod all in retaining the identity. I for one feel that regional language of a particular state should all be made a compulsary subject until a person finishes off his masters degree, in any educational institution pertaining to a particular state whether the school being run by state or central governments should not matter with the geographical location calling the shots...Else it would only breed bits part linguists, that can converse in two or three languages but with zero comprehending skills.
If a kid is fascinated by Bollywood and punjabi lasses, he can make his choice of learning Hindi, dont think any one would say no.