> What makes you think the usual precautions (cell lines, animal models, phase I, II, III trials) will not be taken as per usual?
You've skipped one of the usual phases, which is Phase IV trials. Cell lines -> Phases III trials aren't enough to catch long term side effects, because they are all done quickly.
An experimental mechanism passing Phase III trials is insufficient to prove it is safe. It proves that any negative events aren't evident quickly. The drug will probably prove to be safe in time, and I don't mind if people want to take that risk, I'm pretty free market. But it is, nevertheless, risky. We can't be confident that the body isn't using the same trick somewhere or somehow for something delicate that breaks quickly and shows symptoms slowly - biology is complex.
If the mechanism is novel then it wouldn't be suitable for a mass rollout to deal with this pandemic.