I find it bizarre that some have suggested that pilots should not be able to leave the FD for a toilet break - like they have to condition themselves or put themselves through some sort of diet plan which equates for their flight times and sectors. If you aren't, then you probably aren't running at 100% anyway as you aren't drinking enough and are likely dehydrated, which compromises your performance and decision making.
No matter on your working environment, its a basic human right to be able to go to the toilet or have a quick freshen up break. On some flights, especially on budget carriers from my observation as a well flown PAX that the crew don't even get to leave the FD on turnaround once they've finished doing checks and then start to do their pre-flight checks again for TO.
As much as we want the perfect solution for everything, there is no such thing. There will always be a risk, no matter the measures put in place. I wouldn't deem the FC going to the toilet as an acceptable compromise even as a PAX and 3 crew FDs are a thing of the past except for some very old soviet airliners and LH/ Training
In a time when they want Pilots to do less and increase automation, there's no way on this earth that a 3 crew set-up will happen, especially on short haul.