Nothing. It's a monopoly in a market that is heavily network dependent. Without interoperability, it's infeasible for a new option to become viable. Though I'm sure you know this already and just believe that monopolies are alright for consumers.
WhatsApp is the core of Indian society. It's used by the government, all businesses, and everyone with a smartphone. It's used for payments, customer service, scheduling doctor's appointments, etc. You literally cannot replace it with something else anymore.
But in this case, how exactly does Meta prevent people from India downloading and using another messaging app?