I don't want to get in a long discussion about it but in a nut shell, the BOP is your back-up. Your first line of defense is a good well plan, proper mud weight to give you the head pressure to hold an in-flux down. When you run casing in the well you lock the casing down, then you cement the casing. After that you test the casing and cement. ( 2nd line of defense). If that fails then you can close an annular packer on the pipe, of which we have two, or you could close a pipe ram, of which we have three ( 3rd line of defense). If that fails than you can close your shear rams and cut off the pipe. ( 4th line of defense). If that fails and you have gas in the riser now then you can close your diverter packer at the surface and divert the kick overboard. ( 5th line of defense). So we could continue to add back-ups on top of back-ups until the system is too heavy and complicated to be practical. The root of the problem is operator error. It's just like my work-mate, he has destroyed two Dana 60 differentials behind his N/A Dodge Ram 5.9L magnum. That's how humans work, you give us a system and we can find a way to break it.