The premise is that if a frog is put suddenly into boiling water, it will jump out, but if the frog is put in tepid water which is then brought to a boil slowly, it will not perceive the danger and will be cooked to death.
The story is often used as a metaphor for the inability or unwillingness of people to react to or be aware of sinister threats that arise gradually rather than suddenly."
In the case of Apple, at some point when they do something you really dislike you might be too invested into the ecosystem and it will be easier to just accept it instead of leaving.
But if you were not on their ecosystem already and were thinking of switching to Apple you would just think "Yeah, I really dislike that new move, I'm not buying into it".
It's just a metaphor. It's not a law of nature, nor is it even a GOOD metaphor.
https://www.theatlantic.com/technology/archive/2006/09/the-b...
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The New Psychology (1897): "a live frog can actually be boiled without a movement if the water is heated slowly enough; in one experiment the temperature was raised at a rate of 0.002°C per second, and the frog was found dead at the end of 2½ hours without having moved."