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I agree with you that the value of the coins in your mobile wallet shouldn't exceed the value of the device itself. The problem is that you can have $500 in Bitcoin, for example, and in less than a year the price rises and you suddenly have over $1,500. I'm not a big fan of moving my coins if it's not strictly necessary, but in cases like the mentioned one shouldn't be lazy and should move at least a part elsewhere.
It seems very logical, but from my own experience I will tell you that there is a kind of psychological barrier that makes you leave things as they are, call it procrastination.
IPhone 17 is around $1700 to one device, those that mean I can keep $1600 on my iPhone and feel safe? This analogy doesn't make too much sense. I think if we are having funds that we are not going to move from one wallet to another often like deposit to exchange and withdraw, it's better the money remain idle on offline wallet until the coin is ready to be spent.