We'll never know. If it works then Covid-19 will be a footnote in history, a temporary problem that was quickly solved. We won't know if it would have become an extinction-threatening event or if it would have fizzled out on it's own, or anything in between. If it doesn't work then YC's input wasn't important and didn't help.
This take is too pessimistic. Eventually we will have enough data to project what could have happened without treatments like this. We will know the rate of infection, the rate of death without treatment, etc. There will be many different groups globally trying to find out these answers and apply them to preventing future pandemics.