There is almost no chance of that happening. The amount of time someone occupies that space is so small it doesn't matter. If someone is using the bathroom right as the cabin experiences an explosive decompression, thy could be injured if it happens right before they can open the bathroom door. If they manage to open the bathroom door, that door will take the majority of the impact. And then even if that happens, half the time their back will be facing the door causing their back to take the impact. Only 25% of the time of an already very small window will their face take the brunt of the impact. There are much bigger risks we take. If you are worried about that, why even bother have FAs walking around the cabin?; Everyone should be belted into their seats at all times.