Boiling the Frog
Gradual change going unnoticed until it's catastrophic — from the (biologically false) fable of a frog in slowly heating water. Applied to scope, costs, and cultures in decline.
Heard in the wild
“The service quality dropped so slowly nobody flagged it — boiled frog.”
Tone check: Cliché
The frog would actually jump out; your org might not. Metric baselines are frog thermometers.