It's about who gets the cards in the first place. If India can undercut it'll just be more indians instead of having diverse backgrounds from around the world. As diverse as India could be there are other reasons to pull people from other countries fairly.
This seems exclusively to increase Indian immigration rates and not actually improving diversity of green cards given out. (If you want to play identity politics)
Genetically, Indians are the most diverse ( next to entire African continent ) than any where else. Religion wise (ALL major ones and many, many minor too), linguistically, culturally too.
By any objective measure group of Indians immigrant will be way more diverse as an immigrant population, unless you define diversity incomplete until a favored group of people is not highly included.
In a highly capitalist world, there's no free lunch; surely most of the immigrant must be bringing great value?
The question is, which of these two seems better to you?
* Someone comes to the US, works 10 years on an H1-B, and then goes back to India afterwards.
* Someones comes to the US, works 10 years on an H1-B, get's a green card and stays in the US.