The reason India never allowed US military bases on it's soil is due to its ego which want to be seen as a superpower,and that ego will not allow India to let US have any military base on its soil,no matter the gain or loss.
Downside of the move in my opinion is we will see a decline in our relationship with Russia who won't trust us and our access to top of the line Russian weapons will be permanently shut down.... Russia won't entertain and trust us under USA military presence....
Another downside USA after seeing our relationship sour with Russians will be more dominating as they know that now we will have to depend on them to large extent in the absence of Russia.....
Then again if there is a war between USA and China over Taiwan and if USA wants to attack China from western borders using Indian bases we won't be able to say NO..... by keeping military in India If they are guaranting our security in case of China Pakistan attack then we too can't say no if they want to use our bases in Taiwan war.... This will be the price we will have to pay.... These are all downsides and there will be many more.....
But many US allies are paying this price and living life peacefully without a fear..... Then why not India......
Best thing is with US military presence in India we can reduce our already very small defence spending further and divert the money towards development and well being of the people.....
And yes USA 7th fleet base on Andaman and Nicobar islands...... What a deterrence if China plans anything in Indian Ocean..... QUAD has started taking shape already....
Atleast there are some Indians that realize India is not at all in a position to take on China,it will likely be a spectacular mow down,so that's a plus point.
But,the loss incurred from an US military base far outweighs the gain;
1. China didn't treat India as its nemesis,just an unfortunate neighbor they had to deal with. But with such a move India would have laid itself bare before China as the strategic enemy and just an extension of US imperialism that CN had to take down,and that's not a good position to hold before the future superpower that will have 3-4 times the economy of the USA.
2.India would have send the signal to the world that it have folded before Chinese might( not that anyone believe there was even a comparison other than population size) but still, it's a win for China ,they will take India even less seriously and teat it in a more insolent manner.In reality,US is no deterrence for China against any punitive actions on India,for when it feels their core interest is threatened, ,they already went to war with the US ,twice and China got what it seeked in both occation.
3. Regardless of all the Quad Quad debacle,India needs China for its industries and development,India has to rely on Chinese supply chain be it in Asia or Africa .India can't seriously launder billions from China led AIIB while waging a cold war against China in concert with the US either. Indians should realize one thing ,US isn't funding India's development, Infrastructure or serious high-tech Industry,that's not how US rolls.US might aid the call centre sector and such but that's it.
4. India is US's one of the main competitor (at least in gdp,due to its seer size) in future , after China had taken the unchallenged pole position in the economic sphere ,US knows that very well,hence the US will try to take over India( like in Latin America) before that .
US military base is not something simple as black & white;it's a pandora box that brings with it a plethora of issues. Indian media,corporates,thinktank etc, has already been co-opted by US agents ,they have great sway on Indian elections.But with a resident US military ,India would be run amock by regime change expert CIA ,India would have been again re-colonized by the West.India isn't capable and sophisticated enough to deal with a cia's infiltration techniques,and India can do nothing about it while being so reliant on US after making China as their enemy in the follow up process.
So gradually ,India would get hijacked by the US like Australia,but at a more severe lvl,and the nation will start to play deputy sheriff on behalf of the US at the expense of their own gain,just like Australia,India would then recede to real 3rd world status, probably balkanized thereafter.