This blog post misses entirely that this has nothing to do with the unstable channel. It just happened to only affect unstable this time because it gets updates first. If we had found out about the xz backdoor two months later (totally possible; we were really lucky this time), this would have affected a stable channel in exactly the same way. (It’d be slightly worse actually because that’d be a potentially breaking change too but I digress.)
I see two way to “fix” this:
Throw a shitton of money at builders. I could see this getting staging-next rebuild times down to just 1-2 days which I’d say is almost acceptable. This could even be a temporary thing to reduce cost; quickly renting an extremely large on-demand fleet from some cloud provider for a day whenever a critical world rebuild needs to be done which shouldn’t be too often.
Implement pure grafting for important security patches through a second overlay-like mechanism.