Understood on the ground cloth issue. It seems that you would damage the bottom though.
Rain is the issue; otherwise, you could sleep under the stars--- bugs aside.
Weight seems to go up in proportion to waterproofness. I wouldn't be adverse to a little more weight if traded for a simpler design.
Agreed on many UL bivies being sleeping bag covers to use with a small tarp to catch the spray and cut wind. I have looked at all the scenarios for using a tarp with a bivy and my Gatewood Cape trumps them all for weight and complexity, not to mention the fact that I already own one. I have a poncho shelter too, but that calls for as much complexity as a big tarp.
Hooped bivies approach UL solo tents in weight and cost. Bivies with pole designs are event more tent like. I tried a Black Diamond Lightsabre, which had all the drawbacks of a tent and a bivy--- weight, complexity, no room, condensation, and cost.
I have an REI Minimalist bivy. They have approached the water into the face opening issue by putting a reversed storm flap across the chest to drain water off to the sides. The face opening is mostly exposed.
I wonder about using a waterproof/ breathable bivy with an umbrella to help at the head opening.