Going back to Apple pencil, I agree the battery should be modular, not soldered to board, because that makes your $100 device automatically obsolete in a few years.
Think about it, you can use a Macbook with degraded battery plugged and that'd be fine. What will happen when your pen holds only half the charge it originally did, lets say 50-70% capacity, you'd be writting/taking notes and battery would die constantly. And you can't replace battery, will need to buy new one.
I think the criticism is warranted and going forward I hope they do something similar to the pen in Surface 4: