While it's true that you can't replace one cell, you also don't need to because the pack is designed to lose individual cells without significantly impacting the range. It's not like they're not all in series or something. They're paralleled in medium groups (72 on original model S) with individual fuses, so a single lost cell cuts the capacity of that one group by 1/72. Active rebalancing can transfer charge between groups, so it's hardly noticeable.