This has been explained earlier to you. That's not an indication of Indian economic decline, but an indication of the growth of others.
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I made up the figures, but this shows the principal. Let's say the world is divided in four countries, who have in 1600 an equal GDP, so each has 25% of the total GDP. All countries increase their GDP, but certain countries increase it more than others. Now the smallest GDP is only 2% of the total. But it didn't decline.
Off the bat i want you to realise that I don't:
1. Believe that India industrialized before the British, industrialization is the process of adding chemical energy to industrial processes, what India had before the British rule was a lot of cottage industries that produced goods on a large scale.
2. Believe the infamous 45 trillion dollar loot, i got no idea how they got that figure from, like zero idea.
3. Believe British industrialization was because of India, British industry was already well established before the crown took over the administration of India, did India help the already established British industry to achieve lots of profits? Yeah absolutely but industrialization already was well underway.
Which is a great segway to my thesis, British incompetence.
Now why did India and the British economy decline in importance? As time went by. My reasoning are as follows:
1. The British didn't really industrialize, they did so partly. They never went up the value chain because of the poor human capital. This allowed the German and US manufacturers to outcompete the British one.
2. The British invested a lot of the money earned from the industry in ventures in other countries during pax britania, this lead to a decline in investment in domestic market as capital went out into the colonies and other places. This caused stagnation.
3. India never expanded its agricultural production because of poor land laws which dis- incentivized investment in land and capital necessary for the expansion of agricultural production and this is primarily the fault of the British cause it was they who introduced the system of land tenure to secure political power in India.
Indian agriculture could have modernized and could have drowned out competition in Latin America, and could have made the British and Indians filthy rich but the investment went to south america and africa where a lot of the venture failed
4. Britain costs of being a global policeman during Pax britania: no need to elaborate.
5. British throttled the infant indian industries, even the British East India company was interested in industrializing India to finance the conquest or to reduce the burden but London had other ideas that were stupid in retrospect. This lead to India's economy being primitive, agro based with no significant industries.
British administration in India was corrupt and often took bribes from Indians and foreigners which lead to a horrible system of patronage that countinue to this day.
It is because of these factors, combined with the fact that both Indian and British economy was linked that both countries did not see any significant industrialization during the second industrial revolution. In the end Britain got what it deserved for destroying indian industries, it's relegation to a status of a second rate power which it is now, a fate which they could have avoided if they would have let India industrialized