If you drop a frog in a pot of boiling water, it will of course frantically try to clamber out. But if you place it gently in a pot of tepid water and turn the heat on low, it will float there quite placidly. As the water gradually heats up, the frog will sink into a tranquil stupor, exactly like one of us in a hot bath, and before long, with a smile on its face, it will unresistingly allow itself to be boiled to death.
(From your own link, the story is just a myth that’s not supported by modern scientific evidence. But the point still stands.)
From that same link:
"These modern biologists, however, did not produce any evidence contradicting Fratscher's results since they did not test such slow water-heating as in Fratscher's experiments."
Sounds undetermined whether they croak or not...
This is just a metaphor and not meant to be taken literally. It's about how the masses get used to poorer service gradually. Had it happened instantly they'd protest heavily but doing it slowly isn't obvious.