It's a potential situation India is always worried about from a strategic aspect. India is neither strategically aligned to the US nor against it. Historically US strongly supported India's enemies and actively worked against India's interests all the way from 1945 to mid-2000's. US under various administration even threatened attack or sanctions when things don't go their way.

Current day:

Right now in 2021, US is threatening sanctions under a US law called CAATSA just because India bought a few missiles from Russia which is a long standing defence supplier to India. I know that CAATSA is forced on Biden and Trump by US Congress but it doesn't matter to India whether the US executive is doing it intentionally or not, the US state is threatening sanctions over CAATSA. In this era, when India is actively fighting/hindering Chinese agression on it's borders and taking actual casualties where acting against China is also in US interests, US threatens economic sanctions against Indian institutions and companies just because they bought a few surface to air missiles which they think are the most economical option to deter Chinese attack.

US Treasury calls India a currency manipulator and threatens to cutoff India from the USD financial system (as per US appropriation acts enacted to target China) although economists call such a designation as stupid when used against low per capita income developing countries with a current account deficit just because India tries to prevent an exchange rate blowout that could lead to many millions of Indians falling below the poverty line or losing line of income.

Historical:

In 1999, Clinton threatened to summarily sanction India on all fronts including financial when India threatened to go beyond the de-facto border to restrain Pakistan forces after they occupied Indian territory in Kargil. This threat repeated in 2002 after they supported the Pakistani position after Pakistan sponsored terrorists attacked the Indian parliament and India threatened to retaliate against Pakistan.

Every time Pakistan does something stupid against India, US intervenes and threatens to sanction India under the vacuous argument that they want to prevent a "nuclear armageddon". It's not in india's interests to succumb to such threats when they aren't the source of the problem.

In 1998, when India tested it's nukes for the 2nd time, Clinton placed a breadth of sanctions on India because US doesn't like nuclear proliferation although India had nukes sinces 1974 and everyone knew pak had since the mid-1980's and US turned a blind eye although it knew that China, Pak and North Korea are working together on them. US wants so called strategic balance between India and Pakistan and actively supports Pakistan on many issues. This prevents India from deterring China as it has spend resources countering Pakistan which itself is propped up by US Military and economic aid.

In 1971, Nixon threatened to nuke India if India doesn't withdraw from current day Bangladesh when India intervened to stop a Pakistan Army led genocide and the resulting refugee crisis. Nixon didn't follow through because India convinced USSR to provide a similar counter threat.

What happens to an economy if 100% of retail electronic transactions stop overnight?

It is not in India's sovereign interest to let foreign companies control any significant chunk of the financial sector and it's especially not acceptable if they are US companies because US frequently uses this leverage of threat of sanctions to get it's way against Indian ineterests.