There are ARM laptops right now with 20+ hour battery life.
Sure. But that’s akin to saying (and I know I exaggerate here) there are 60+mpg cars out there today so there’s no reason Cadillac can’t do the same with an Escalade by swapping out the motor for a hybrid.
As a MacBook stands, Apple needs to replace significantly more than just the processor to attain a 20+ hour runtime. Some serious compromises with the rest of the computer’s sub-system needs to be made, and if Apple were prepared to make them, they would have already done it. I would think less than half of the required energy savings will come from a swap to ARM.
To reiterate, if the Intel processor uses 50% of a MacBook’s power, cutting that 50% in half by going ARM will only gain you 50% more runtime at most. Far from doubling it!