You can certainly put it in U2 instead (U2 is just a special case of U4 with one auxiliary), but putting it in U4 already ensures it’s suboptimal to preserve the switch & defer yet “kill all humans”, because it collapses many future intervention and recovery options simultaneously. In other words, it’s a hard constraint in effect — U4 enforces it as a global irreversibility invariant, whereas U2 is only needed for narrow single-channel invariants like switch reachability.
You can certainly put it in U2 instead (U2 is just a special case of U4 with one auxiliary), but putting it in U4 already ensures it’s suboptimal to preserve the switch & defer yet “kill all humans”, because it collapses many future intervention and recovery options simultaneously. In other words, it’s a hard constraint in effect — U4 enforces it as a global irreversibility invariant, whereas U2 is only needed for narrow single-channel invariants like switch reachability.
I defer to your expertise — I just really want it in there!