Be aware that it would never delete any date range archive. To make it more useful for more people maybe it makes sense to define something like purge_date_range_archives_after_X_days = 1 . Name would be to defined. We would pass this value to the method and instead of generating the date "yesterday" hardcoded we would go back by the number of given days which is by default 1 (for yesterday). To disable it completely as you want to we could use the value -1 or 0 or whatever. Or one simply specifies a very high value eg 99999.

BTW: You also want to rebase master I reckon. That version of Piwik is about a year old.