Most choices are not cake or bread.

Most choices are bread or going hungry.

Or a new bridge instead of an old failing bridge or no bridge at all.

The fact that the bridge failed catastrophicly rather than ductally suggests the fundamental problem was systemic in the design and the mode of failure was from unanticipated forces.

What I mean that if the force was 8x the design load 2x vs 4x safety factors would be irrelevant.

The idea is to work out the maximum design limit, then add a factor of 2x or 4x to that to allow for things you can’t predict or model well. If the design is relying on some part of the above of the 2x to work, it’s not really a safety factor.

I’m a mechanical engineer and figuring out the appropriate strength a part needs, then making it twice as strong, is a common tactic. You can’t account for material differences, little casting voids, or just your math being off a little by the approximate methods we use to calculate stresses. The more perfectly we can model and then repeatedly destructively test a part, the closer we can get to a safety factor of 1.