There was some note (can't find a link) about VW updating their software to alert of pack issues and bring the car in for a specific cell swap. Maybe if it gets bad it would affect nearby packs, or there could be a safety risk?
So its basically a numbers game to get lower cost to the manufacturer within 8 year warranty:
- Battery failures * full replacement cost of the battery pack
- Cell failure * cost of a cell replacement + presumably higher BMS/initial cell manufacturing costs
Of course for the end user, replaceable packs is preferred. But, you'd want to know reliability first.