What are the positive practical (social and economic) implications of categorizing English as an indigenous language?

Any negative practical implications?

Well, in one sense yes. There are a significant number of native speakers in India, as well as a distinct Indian variety of English. Most speakers are L2, but despite its colonial history, it has a neutrality that Hindi or Tamil hasn't been able to find, and also practical international uses. It has also established itself in a way that French and Portuguese failed to.

English is indeed colonial in origin, but it has been around in India for hundreds of years and outgrown that role.