While I only have one deck myself I can see why it could be convenient to be able to mass-delete. Deleting one by one is really quite tedious when there are more than ten or so things to delete, especially with "Are you sure?" confirmation pop-ups.
Realistically speaking, it's rare that all decks become obsolete after a new patch. Maybe some players just automatically throw everything away and start over. For me, I rather have a sorting order feature. Then I'll just move the new (or still viable) decks to the top. I want to avoid removing old decks because sometimes I still use them as templates for improving decks.