Sanskrit, or more correctly, pre-Vedic and Vedic language, was not a code language, a secret language. It was the language of the people of Central Asia at that time. That is why Avesta was written in it. It was new language when it came to India but quickly displaced the old languages. The prakrits, Apabramsha languages are still spoken in many regions of India - Maithili, Awadhi, Bhojpuri, Dingal in Rajasthan, Gujarat and Sindh (
Dingal - Wikipedia), and many more. All such languages are basically prakrits, derivatives of Sanskrit as the people spoke it.
Which is the king that you mention as assassinated? Sure, Hinduism became the majority religion in Gupta times. Adoption of Sanskrit was not the reason for decline of Jainism nd Buddhism. Jains remain in North India even now and Buddhists were in North India till the sack of Nalanda around 1,200 D by Bakhtiar Khilji. Nalanda was supported by Hindu kings all through the history. Jainism and Buddhism reached South India much earlier than that and at one time they were prosperous there. I put reason behind the decline of these religions as too much ascetism and academism. The simple village folk were not interested in ascetism and did not understand the complicated philosophies. They found Hinduism much less demanding and easier to follow. I do not know of any major genocide. Sure, there are some stories in Buddhist and Jain literature with which they tried to explain their waning influence, but there is no supporting evidence for it.
The problem is that your bias against North India, Sanskrit, Brhmins and Hinduism is clearly showing up.