What if last backup does not end successful and backuped system crashes? You do not have valid backup in catalog, lucky if image was stored on not rewrited tape. here question, does it possible do not expire last success backup until not created new?
Any backup that is not successful is removed from NBU catalog. The next backup will overwrite the data written to tape during unsuccessful backup. There is no point in keeping partially written backups. It can never be considered to restore from.
You can only restore from successful, status 0 backups.
If you are concerned about the possibility of a backup server that may crash, consider clustering your master server and creating STUG if you have more than one media server.
Investigate backup environment to eliminate Single Point of Failure.
Look at retry settings in Global attributes.
Read through NBU HighAvailability AdminGuide.
Compare backup intervals and time taken to complete backups with your company's RPO and RTO.
If last successful backup is not good enough to satisfy RPO, you will need to look at alternative data protection methods.