It is indeed true that if India needs to have supercomputers urgently, it should buy them off the shelf.
But it is also true that India needs an indigenous supercomputer capability. That way, India will be able to have as many supercomputers as its people can build, not limited by the willingness of foreigners to buy Indian products so as to give India foreign exchange.
After all, India, like any other country, wants an economy like that of the United States, Japan, and other Western industrial nations, based on ownership of intellectual property, not one based on exporting resources, or the products of low-paying menial labor.
How one constructs a world economy with all generals and no privates, of course, is a question... but if India has its proportionate share of the commanding heights of the economy, even if there is also employment for farmers and factory workers and miners in India as well, that would be enough for contentment. Ultimately, that is the goal, for every country on Earth to be equal (as well as free and democratic, and with equality for all ethnic, linguistic, and religious groups within its borders).