When I started technical dive training, I was required to carry a spare mask. When I took cave training, my instructor did not carry one. When I asked him why, he said he'd been diving and instructing for 40 years, and he didn't know anyone who had ever lost a mask during a dive. He would take the risk.
I once had the hinge where the strap connects to my mask break off (don't ask--accident setting up a platform). I then did a 30 minute dive without a strap on my mask with only minimal trouble. If you do "lose a mask," it usually doesn't disappear; it is usually knocked loose, and you can grab it easily before it goes away, usually before it even leaves your head.
If through some incredible chance you actually do lose your mask to the point that you have to look for it, that is what a buddy is for. That's why the no mask swim is part of instruction.
So, IMO the odds that someone will ever lose a mask and have to search for it with eyes open and no buddy present are so incredibly remote that I doubt I will ever meet anyone in my lifetime who has in turn ever met anyone who has ever had to do it. If that happens and the diver needs to open his eyes under water to look for it, then I bet he will be able to do it even without having practiced it in the OW class. Opening one's eyes is not a difficult skill, and anyone can do it if the need arises.