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.