Yes, napping in the flight deck is a problem. The point of having two pilots is redundancy, if you incapacitate one of them, what’s the point?

It’s not an outrageous requirement to be conscious during a duty day.

Sure, when you are in cruise, it’s a low threat phase of flight, but if you need a nap in cruise, chances are you were fatigued when you departed.

A much better system is to allow pilots to remove themselves from duty, without repercussions, when they are fatigued.