I am honestly quite surprised by all the negative reactions in this thread to this decision.

How many people here have actually flown while wearing a mask? I flew from Australia to Europe in March while wearing a mask and it wasn't too bad. The only (major) downside was that my ears started to hurt after about 8 hours of wearing the mask. I don't think lots of people are taking trips this long right now. And if they are, there are ways to prevent painfull ears.

One other argument against masks (at least in the Netherlands, made by the RIVM) is that people wearing masks will not keep their distance. The negative effect of not keeping distance would be bigger than the positive from wearing a mask (Source (Dutch)). However, when flying it is impossible to keep your distance anyway, so this argument doesn't make sense to me.

In my opinion the downsides of wearing masks are so small that they definitely outweigh the possible downsides. So unless anyone has some strong evidence that wearing a mask increases the risk of transmitting the virus, I applaud this move by KLM and ask myself why they did not do this earlier.