I once built a Yocto system that had both... We'd use our package index for quick hotfixes, and push a full OS image to the A/B partition for larger, riskier changes. It was nice to have options.
i was going to just comment "but systemd" but i just found out debian ostensibly supports uninstalling it and installing openRC instead and that makes me like debian more. I use debian for generic VMs, prod VMs are 50/50 gentoo and ubuntu. I've been messing with Devuan too as my primary linux VM on my desktop PC. At one point i had it booting to fully logged in in around 8 seconds (after the bootloader selection thing.) unfortunately i broke that feature so now it takes like 40 seconds. But it is also openRC (there's a pattern here)