When Boeing was developing the 767 (~1978-1982), the FAA released their study that said that a 2 crew flight deck was just as safe as 3 crew. Boeing went to the launch customers and asked if they'd fess up $500,000/aircraft for an EICAS equipped 2 crew instead of a 3 crew. Every customer immediately responded YES! (except Ansett - and that was apparently union related). They said break-even for crew costs was just over a year - and that was early 80's - figure ~4x that in today's money. The costs to do it are largely non-recurring - so once the airframer has done the development the delta cost to each additional aircraft is small.
Not saying it's the right thing to do (I have mixed feelings), just pointing out the economics.
BTW, while the presence of another pilot may help discourage a potentially suicidal pilot, if a pilot decides they really want to crash the aircraft, having a second (or third) pilot is unlikely to stop them. Going full nose down close to the ground will suffice (e.g. Atlas 3591, although I don't think that was intentional).