With Summer'18 orgs are limited to 250 MB of stored debug logs - Store Larger Debug Logs with Changed Log Limits
When your org accumulates more than 250 MB of debug logs, we prevent users in the org from adding or editing trace flags. To add or edit trace flags so that you can generate more logs after you reach the limit, delete some debug logs.
As it isn't generally good practice to delete logs that others are creating this puts the onus on the developers within the org to coordinate on the deletion of excess logs.
In the majority of scenarios, the debug logs are only required for a short period of time after they are generated.
Rather than retaining all the non-system debug logs for the full 7 days add an option to the TraceFlag that indicates the generated debug logs can be freely deleted as required. Then, if the org every reaches the 250 MB debug log limit the oldest debug logs that aren't flagged for retention can be automatically deleted and the new logs created.