1. Govts can (and should) regulate -exchanges- who are setting the exchange rate at the same time as offering to be the bank for poorly educated crypto holders, so they can use folks money to buy low and sell high.
2. Govts (already) make it a legal obligation to declare and pay tax, crypto is no different than dealing in cash.
People who break the laws of their country should be tried under those laws, ie: tax evasion.. this isn't new to crypto.
What the RBI are saying is they object to crypto removing their ability to.. (see point 1.) control the supply. When they talk about managing the economy, what they mean is 'their ability to print money'. Furthermore they argue their ability to make up the rules as they go is enshrined and justified by the notion that 'that's how it's always been done'.. well welcome to technological advancement? Crypto is FAR from perfect right now but it is based on a premise that a truly fair and un-cheatable system is possible, and that has govts terrified.