I think we're fast approaching the point where we're repeating arguments already made and talking past each other but I'll give it one last shot.
You believe a national language is conducive to national harmony and understanding. Sure I'll concede the point but so is say a uniform national religion. So let's change the context a bit and repeat your argument.
I expect you'll retort religion is more central to people's identity. Sure but it's not to mine and I expect language is very central to the identity of say Tamils and Kashmiris and Balochs. They don't want to learn Hindi or Urdu. So don't make them just because you have some vague notions that it promotes 'harmony and understanding.'
I will concur with the bolded part based on my observations.
There is a large group of south Indians at my work place from Chennai (Tamils), Bangalore (Kannada) and from Kerala (Malayalam). They are smart, open minded, and tolerant people. All of them hate the idea of having to learn Hindi. Before I had the fortune of meeting them, my idea of India was colored more by Bollywood, popular Indian news media, and other Urdu/Hindi/Punjabi speakers -- basically an opinion of India through a lens only focused on North India. My views on India as an aggressive Islam hating right wing Hindu -jingoism nation was through the North India lens which had similar "Hindi is the national language of India" jingoism. My opinions changed once I interacted with my South Indian coworkers.
Yes, Baloch or Kashmiris could have the same objections to another language forced upon them just as Tamil or other South Indian people.
BUT there is a subtle difference here though between India and Pakistan.
In India - Native Hindi speaking majority is forcing Hindi down the throats of non-native minority.
In Pakistan - Native Urdu speakers are a minority. Non-native Urdu speakers insist on Urdu as a common thread among other non-native Urdu speakers (while subtly also including the native Urdu speakers).
Why this distinction is important - If someone forcing a language upon me but the enforcer does not have to put in any extra effort in learning a language but me the enforced person has to run that extra mile to appease the enforcing majority then it is unfair for my linguistic minority group. If my minority group is either native to the land or has sizable demographic/political voice then I will definitely resist.
Many people who view things from their majority group lens (language, religion, or anything else) seem to miss the above simple logic until we get exposed to another way of seeing things.