The persistence of the canard that India was once a rich country is understandable. A quick glance at the simple graph illustrates very clearly why it is easy to conclude that India was rich.
This piece has loads of graphics — more than any other piece I ever wrote. No arguments here, just a lot of data-based graphics.
Here’s the TL;DR version: India is not rich today, and really never was rich in the past. In fact, all countries were poor until about a thousand years ago. India had a large share of the world GDP because India’s population was a large share of the world population. India was not rich. It was merely big. Just like it is today: big and poor. Like China used to be just a few decades ago.