Suggestions on replacements for the boiled-frog metaphor:

"The cat litter box is a good substitute and there are plenty of similar examples if one considers sounds. A succinct example of this is from the movie the Blues Brothers. After Elwood picks up Jake from prison, they go to Jake's apartment in Chicago. Right after an El train rattles by, Jake asks Ellwood, "How often do the trains come by?". Jake responds, "So often, you don't even notice."

"A similar situation occurred with my grandmother. She and her husband bought a house near LAX in the 50s, when I'm sure the occasional airplane flying over was a pleasant distraction. At the time of her death in the 90s, LAX had 4 runways and their house was bombarded by the noise from constant aircraft that seemed to be flying just a few feet overhead. Anyone walking into her house would have wondered why the TV was so loud, until the next takeoff or landing occurred.

"Two more frog equivalences. Ever work on another person's computer and find that it operates brutally slowly? No one ever seems to notice that almost day-by-day loss of performance themselves. Easy to understand, but I can't think of a catchy shorthand.

"So finally, my nomination for the replacement, an old person driving. They never notice the degradation in their driving skills until some unsuspecting passenger or pedestrian has a (hopefully only) near death experience."

More to come.