How do you design a switch input to a MCU for ultra low power consumption?
Turning the internal pull-up (or in your case, pull-down) resistor off is indeed a way to minimize the power consumption when a switch remains asserted against that resistor.

However, by itself that is unlikely to be workable.

What you need to do is turn off the resistor, then put the MCU to sleep with a scheduled wakeup interval at which you will wake up, briefly re-enable the resistor, and check to see if the switch is still closed.  If it's still closed, disable the pulling resistor and go back to sleep for a while before checking again.

You may also want to use an extremely high value external resistor to bias the pin in the unconnected case, since a floating input could otherwise end up near the logic threshold voltage, and cause a degree of shoot-through of the upper and lower input transistors, leading to excessive current draw.