>What I mean is that a big refactor might take 9 months to do atomically whereas it could have been done in 3 months as a massive single PR.

My experience with these 3 month-long PRs is that they have a tendency to not get finished. Then they get stale. Then they get abandoned. Then somebody starts another one.

In one case it was finished, but the CTO was just too afraid to deploy it because it was big and it might break something. So while we were waiting for the right deployment window it got stale, meaning more conflict fixes....

I also think most people overestimate the amount of extra work involved in breaking up a refactoring. It's usually no more than ~20% more coding, and that extra work almost always pays for itself by reducing deployment risk.