There is no need for digital mass surveillance to enforce a quarantine. It's been done before computers were invented.
How do you intend to achieve this miracle? Will the plan involve Maoist policies? How will the plan be hardened to prevent its being used for Maoist policies? Any mechanism that could possibly coordinate a worldwide lock-down of everyone is eventually going to be used to implement a "common sense" policy that starves ... these days it could be a billion people if coordinated globally. That is a much worse idea than the COVID pandemic which, apart from the lockdowns, was nowhere near as bad as a system that could allow global coordinated action to disable the world economy in coordinated fashion on demand.
The thing you're avoiding talking about seriously is the part that isn't already a certainty. Ie, the entire question.
It's a certainty because it happened. R_t fell under one as a result of NPIs, not immunity.
You don't have to keep R_t under 1 forever. Just for long enough that the virus fizzles out. It's possible and in fact it has been done repeatedly.
> However, that would have required very decisive/heavy-handed and coordinated action across the world.
This is infeasible. If we had a system for global heavy-handed coordinated action we'd be much worse off than what COVID did; the political ramifications of that sort of measure would be disastrous. We went in to COVID with economic concerns and a bit of turmoil in the world and came out the other side with a rolling economic crisis and a potentially brewing WWIII. Having a reliable mechanism for triggering that sort of thing would end poorly. Also the US already spies on everyone, having mechanisms to be able to enforce its whims would only make the situation worse.
[0] https://en.wikipedia.org/wiki/Timeline_of_the_COVID-19_pande...