I have watched some projects absolutely explode in LOC added, number of PRs, etc. but I think the more interesting question is: how much of it is directly being done to add customer value, how much of it is churn, etc. you might get some interesting answers.

As so frequently seems to be the case for you and I, we kind of agree but then you drop something that just does not compute for me: "slow is smooth, and smooth is fast" is not specific to "mission critical" systems, it is generally applicable.

As I said in a previous comment, I work on a fairly boring system. Its "criticality" is debatable, but in general we make the same kinds of boring guarantees to our users that even mediocre SaaS products offer: a few 9s of uptime, zero-downtime deploys, etc. AI has made aspects of working on this system easier, but in terms of API surface, how users are using it, how to safely advance its state without breaking existing callers, data migrations across services, and so on, very little has changed.