Sure, Apple could build their Pencil with a couple of easily user-replaceable NiMH AAA batteries instead of the custom-fitted battery pack. However, the AAA's weigh almost a full ounce by themselves, not to mention the compromises Apple would have to make on the svelte design.Wouldn't be an apple product if it wasn't built to have a limited life span to force you to buy the next generation.
Remember when they used to make things with replaceable batteries ?