No known treatment or vaccine. Targets the decision makers (presidents and ministers and military generals are older, and a virus is easier to reach them, than a bullet is). SARS-CoV has super-spreader events, and asymptomatic spread, making it very difficult to contain. It spreads incredibly easily (near-airborne), in confined spaces such as airplanes, but even the toilet plumbing, or shared airco. It offers plausible deniability, by pointing to a natural spill-over event or unsanitary meat markets. It causes enormous economic damage (the economy, not the cannon fodder, being the subject of modern warfare) and cultural damage (tracking and containment is costly and invasive as it damages trust in a free society). It is most readily available to small states and terrorist groups by extracting from live civet cats. Military-aged terrorists spreading SARS-CoV by simply boarding airplanes and visiting hot spots, and not even dying themselves, so they can do it all over again. SARS-CoV in first stages has vague symptoms, similar to other, more common viruses, which would give a pandemic a head start. Pandemics are good PR for fear-based terrorism. The strain on the hospitals is enormous, and military-aged men are too worried to reserve a bed for their elderly parents or their recovery.
More in the vast literature.
> SARS-CoV-2 is complete trash as a bioweapon.
If complete trash, I would not be afraid of Iran being able to press a button and release SARS-CoV-3 for a 2020 repeat. Even if looking at viral bioweapons from the comical anthrax perspective: SARS-CoV-2 killed over 2 million people.
> that the existence of the sample would almost certaintly be known.
Yes. It would make sense that it would be known. And if it would be known, that would probably sufficiently proof the lab leak hypothesis and end this quarrel. We wouldn't need to talk about probabilities much anymore. But that same argument kinda also works against the zoonotic origin hypothesis. It would make a lot of sense that after a year, we found the intermediate host, or patient 0. If there was a clear epidemiological explanation for a zoonotic origin, it would almost certainly be known. Maybe there really isn't.