I agree with your point that if the price of CNG comes close to petrol then this will definitely impact the conversion of vehicles to CNG.
But another important thing to keep in mind is the focus of the entire world on reducing carbon emissions opens ways for other available options. Transition to EVs will take time so until that time using CNG vehicles is more environment friendly as compared to using Petrol/ diesel vehicles. So there are chances that govt would announce favorable policies for CNG vehicles also.