As part of my job, I do risk analysis and mitigation. I agree that it's not terribly useful to stack up the potential harms, hand wave and say that the advice to wear masks was wrong at the time (indeed, the paper reiterates this multiple times). However, going forwards, it's useful to have data that explores exactly what is being impacted and, where possible, to quantify the extent to which it's being impacted.

As they state in the discussion, this doesn't just have to be making a decision between masks and no masks; strategies could be adopted (particularly when caring for the mentally vulnerable) to reduce the impact. Not that I'm attributing this to you but it's disappointing generally when people skim a headline and laugh at the temerity of a researcher to explore something that, on the surface, is common sense.