Technically, the downstream breaker COULD just be a main switch, i.e. no trips. But what you will find is that the only "switch" available in a panelboard is a Molded Case Switch, which is a breaker with no trip elements. However, if you try ordering it that way, you find out that it's a special so you actually pay more, even though technically the MCD could be cheaper because it has fewer parts. It's really all about volume.
But an MCS is available as an option in many cases, specifically BECAUSE you can't predict which one will trip first, and in a situation where down time is thousands per minute, taking 1 minute longer to figure out which breaker tripped is going to cost you more than the difference in the cost of the Main device.