You are correct about purging. However, you can set the number of backups to keep for the "auto" option, in the backup preferences.
I have written a utility to purge the files but would be much happier if it were under Quicken's control
I would not trust Quicken with that responsibility. Better under user control in my opinion.
I keep my Quicken files in Dropbox and have for years.
Do you mean your active Quicken QDF data file - or just the backups ?