Your assertion that Hindi should be declared the national language because it united the people of India during the national movement is based on the rigid understanding of the language. Your argument overlooks the fundamental principle of unity in diversity that was central to the idea of India during the national struggle for independence. Moreover, there is no single version of Hindi; many dialects are being spoken.
The current form of Hindi promoted by the Union government is a Sanskritised version and is not used in daily discourse. If you move across Rajasthan, Haryana, UP, Bihar and MP, you will find different versions of Hindi, profoundly influenced by local dialects. In the face of such difficulty, which version of Hindi will you select as the national language?
The solution lies in encouraging people to learn at least one other Indian language. It would in turn usher in the process of formation of a new link language derived from all Indian languages. Let the lingua franca evolve naturally through consensus and dialogue.