That would be extremely unethical, exposing the control group to needless risk. There is already another drug that helps prevent hiv, it's called truvada. They tested this new one against truvada, and basically learned it's a lot more effective.
Who would sign up for your study? The diligent person already on truvada - would they risk getting placebo? No. Or the person too careless to take truvada? Would they go to the trouble of participating in a study for a 50% chance of protection? No.
Ethics and managing relationships with the public overlap.
But I don't think the particular aspect of "making sure you have enough participants" interacts very much with ethics. Especially when the failure state in question is an honest failure to attract them.
I think it would unethical if the participants in the study were asked not to take any other antivirals.
In that case, someone at high risk for HIV infection could be prevented from getting treatment because of the study protocol. The ethical solution, which the authors of the study chose, was to provide the highest standard of care as the placebo - in this case it was Truvada.
Since the treatment was actually even more effective than the existing standard of care, it was a home run success. Their conclusion is even stronger than it would be if they used a non pharmaceutical placebo, so in retrospect their decision was clearly correct.
