Tesla Warranty replaces at <70%. It isn't true that a battery at 80% is no-longer fit. The article author fails to grasp that there are for example 8,256, 18650 battery cells across 16 modules making up the battery pack, in a 100 kWh Tesla. A single cell may be dead at below 80%, but that's not normally how these packs fail. Normally its total cell failure within the pack, which due to the charging and monitoring system means the cell is excluded.
Obviously smaller packs are less tolerant to this degradation. Is it really any different to a Petrol or Diesel car losing 20% of its claimed BHP after 5 or 6 years?