Resistor power rating and circuit stability
The statement that resistor resistance changes with temperature is true, but only very little (much below 1000 PPM/°C), so the point where you state that at cold temperatures the resistance is so much lower that it consumes more current is simply not true.
A typical 100 ohm 1% resistor is that at 20°C, and it will have a temp coeff of 250 PPM/°C so it will be better than a 100 ohm 2% resistor at temperatures within 40°C of that.
That is hardly enough to change the resistance to change the current enough to exceed the rated power dissipation limit. And if it is enough, then that is the exact reason to derate the resistor, i.e. change to a resistor that can handle the power dissipation without heating up too much.
Changing to a resistor that can handle more power heats up  to less temperature because it can radiate and conduct the heat away better, and it also heats up and cools down slower due to higher mass, so for example the stress due to  repeated thermal expansion is less and thus the circuit can handle more thermal cycles and has longer life.
If there are user operable parts like connectors, a higher wattage resistor can survive user induced faults like short circuits, if it is rated to handle it instead of just rated to handle normal operation.