This hostie-to-the-cockpit rule achieves precisely 0.01% of SFA, the only reason they're doing it is because it doesn't cost anything to implement. Same for moving cockpit doors around and installing loos in flight decks and all these other bright ideas.
It just means the next guy won't do it in the cruise, he'll wait until he's on half a mile finals and shove the stick full forward. Or turn the engine masters off during initial climb. Or, if he wants to do it in the cruise, just stand up to get something from the coat rack and then garrot the skipper before continuing. Or one of a hundred other things he could do. A pilot who's determined to suicide will always find a way.
The best cure would be good, free, medical insurance, backed up by a company policy that will allow guys to take time off and get their head straight without fear of repercussions - or pay out a really good loss-of-licence cover in the event that they can't be recertified.
And it needs to be either government funded or legally mandated so that companies can't wriggle out of it.