performing a medical experiment/procedure on someone against their will is unethical.

Experiment? Certainly.

Procedure? When not doing the procedure will (to continue using my totally made up hypothetical numbers) cost 5 million more lives? That’s much less clear to me.

(Reality is more likely to be “we reckon it might save 5 million lives net, but then again it might combine with the original to make something both virulent and highly transmissive”, but the joy of hypotheticals is I can ignore that).

My point was that the spillover of a significantly more lethal but highly transmissible virus is unlikely, but becoming more likely with each tick of population growth.

SARS-CoV-2, as terrible as it was, wasn't nearly as bad as it could have been: a better-adapted, more transmissible, and more lethal first wave.

And if that scenario were playing out, a human-engineered firebreak, even at the cost of many lives, isn't an obvious ethical wrong. Difficult decisions.