Sure, reasonable point for discussion. It's up for debate whether the cost benefit of discarding usually a lot of training and experience is worth it for a colossally embarrassing fuckup that will almost invariably be successfully punished, deterred, and learned from and practically never repeated. I would suggest that if 'gun left behind' turns into a default dismissal, then the rare case of 'gun left behind' risks turning into a situation someone tries to cover up for hours in hopes they can find it with the resultant risk of a 'gun left behind' turning into 'gun gone astray', versus being an immediate 'oh shit!' with the member knowing they're going to eat some major crap, but there being an incentive to immediately bring the problem forward for help in recovering it.
Absolutely it's an intolerable screwup. It absolutely needs to be punished seriously. I'm not certain that, in every case, loss of employment will best address the problem to be solved.
Food for thought.