you've not toured before have you?
For this [front wheel lift] to happen, you'd have to be starting on a 20% + slope, aimed straight up,and not leaning forward when standing.
I've toured rear load only in Latin America with very steep sections, and it's easy enough not to have this happen.
I guess I can see how you could imagine this happening, but it's fairly easy to avoid this, and it's very rare to encounter such steep hills, and it's easy to start at an angle or almost sideways on road.
Front wheel lift isn't that uncommon; I've done it unloaded on grades less than half of that while just riding along (but not being too careful).
Having said that, there's not much of a lever arm from the rear axle to the center of gravity of most loaded panniers, so your overall center of gravity isn't going to shift very far back when you load up. And since your body weight distribution dominates the c.g. of the bicycle, leaning a bit forward can compensate for the load and prevent wheel lift.
Bottom line: front wheel lift is disconcerting, but it's not that big a deal.