It is dangerous, it has significant side effects. It is worth it for things where it has therapeutic utility, but clinical trials for it in COVID-19 keep coming back showing no benefit.
> A drug with a 10-50% success rate will still save a lot of lives.
It would. But one with no measurable benefit won't.
> This drug already saves millions of lives.
Perhaps; it is certainly useful for treating certain conditions that aren't COVID-19, and it should continue to go to people who need it for things it is effective in treating. No one is saying it's a bad drug, just that the evidence is that it's not a useful drug for treating COVID-19.