This is not a flip the switch tomorrow hypothetical. The fallout of such an action would be huge for everyone, including the US stock market.

Here in Europe, every government and major corporation have recently added their dependance on US platforms to their risk management taxonomy. For the (unlikely) scenario you mention, for the scenario that their company/government somehow runs afoul of the US goverment and this is used as leverage, for espionage reasons, and for other reasons that may have already been in their risk overview (data privacy, compliance, etc) but were seen as manageable but are no longer so.

For some anecdotes: My former employer just moved off of AWS to a EU provider and will likely also move away from Google Cloud for their internal needs, my current employer has started evaluating moving off of Azure at the request of our clients (though they dismissed the idea of moving off of Office 365 internally), and my partner's company (a large corporate) has started prioritizing a transition plan away from AWS.