Every disposition rule depends on an event. The matter closes. The engagement ends. The clock runs from a date somebody can point to. When no system records that event, disposition stops being an operational task and becomes an argument.
The difficult triggers hide in three places. Your policy names an event nobody performs, because "upon matter closure" assumes closure is a status someone applies. Your procedures assume a step that was never anyone's job, so a matter goes inactive and still shows open five years later. And an outside counsel guideline tells you to dispose "after a period of inactivity," which could mean five days or five years. When the standard is subjective, nothing is clearly eligible, so nothing is defensible to delete, and the data estate keeps growing.