Next, do you think the cartels or monopolists who come to control opium supply will show the poor of India more mercy? Do they currently?
You can't compare a local cartel to a foreign, hegemonic imperial power.
In addition, what leads you to assume that the opium trade will inevitably be controlled by cartels or monopolists? Is marijuana production in California controlled by cartels or monopolists[0]? Opium is already produced in India (legally!), and its production isn't controlled by cartels or monopolists. And the plant is both endemic to the region and easy to grow, making it incredibly difficult for any entity to seize control over its production.
[0] Answer: no, despite all their best efforts over the last couple of decades.
As someone who has both studied pharmacology and worked in drug policy, I've seen this common misconception before.
Opiates, as a class, are not inherently any more destructive than any other class of drugs. It's possible to synthesize very potent forms of opiates, but it's also trivial to synthesize or distill very potent forms of other drugs as well, including drugs which are either loosely regulated or completely unregulated.
Opiates are, in certain ways, less addictive than alcohol. That's not an endorsement of opiate drug use, but it does mean that we can't say that "the destructiveness of opiates derives from their very nature" without saying that "the destructiveness of alcohol derives from its very nature", and questioning the implications of what that would mean.
A good example is heroin use in servicemen after the Vietnam war.
https://www.npr.org/sections/health-shots/2012/01/02/1444317...
The findings of the Vietnam study tell us something interesting about the particulars of Vietnam and maybe war zones in general, but I doubt they hold much relevance to the general addict population.
One Kg of Fentanyl is 20 MILLION strong doses (50mcg). One Kg of carfentanil is 50 million lethal doses (at the 20ug ld50 espoused by the RCMP)
So in a country of 1.5Billion people it only takes one bad actor to fuck over a significant part of the world.
Opium was explicitly used as a tool of war by the British in order to colonize China. You can't look at what happened in the 1800s in China with opium without looking at the entire context in which it happened, how it was spread throughout society, how it was promoted, how it was sold, and how it was weaponized.
Legalizing opium in modern-day society (especially given the current patterns of use) is a very different proposition from what happened during the Opium Wars.