On the simple point of no zippers:
Absolutely, this saves weight and makes for a more insulative bag. And it packs smaller. Is it REALLY less versatile? Do you overheat in moderate temps? I'd argue we can get used to sleeping on anything. The Chinese used to use stone blocks as pillows. I used to use sheet-blanket-comforter adjusting the combo to the temps, but my wife wanted a single, thick down quilt. I learned to deal. I toss some of it off my shoulders or feet and that works, unconsciously, for me now.
As for the OTHER reason for zippered bags: when most Everrest climbers are married (or domestic) partners, THEN let's go back to zippers.
Second point on Zippers: When working in a BP shop, we'd order equal numbers of R and L bags. When we were a little heavy on L bags, I'd point out to customers (accurately) that far more people had R bags and if they wanted to maximize the possibilities, they'd buy a L bag.