I might be all wet*, but I wonder about the relevance of bivy bags in an era of ~1# (and lighter) solo tents. One would choose the bivy for...cost reasons?
*and bitten by insects
Only once have I actually seen someone use a bivy sack, that was last summer on a backpacking trip. She came into the campsite after sunset but it was still light enough to see. She was through hiking the trail and making a lot of distance every day. She was camped really light, when I asked her how she deals with packing up her gear on a rainy morning, she implied that if you pack up fast enough nothing gets too wet. It was late summer, bordering on early fall, about 20 miles from Canada so it was far enough north that the weather was cool. And almost no bugs, on that two week trip I only used repellant once. Next morning, I think she was on the trail an hour before I was, I think she was hiking out when I was still working on my coffee.
Decades ago I did a canoe trip with three others. One guy said he would bring his big four person tent. The first night, I decided that it was really tight with four, later nights when it was nice out I just slept outside, which unfortunately meant a heavy dew on my down bag every night. That is the only trip that I have wished I had a bivy along.
I can see a bivy for someone that really values traveling light if they also have a tarp for rain protection like Staehpj1 mentioned. I used to campout under the stars a lot more often when I was a kid, but now I value a tent more. When I was a kid, if you had a nice tent you were clearly of great wealth, back then my friends and I were more inclined to deal with less gear because there was no choice, we had what we could afford.