XF updates it T&C's to say that add-on's/styles uploaded to the Resource Manager that are marked as unmaintained/deleted by the strict housekeeping process have their license changed to GNU GPL. Dev's are aware of this when adding or updating add-on's.
This will ensure fairness for everyone, as it will encourage dev's to sell or otherwise pass on their add-on's if they no longer wish to maintain them, or otherwise make/let them become open source. The original dev does not loose, because the add-on is not being sold anymore and is unmaintained anyway (or they've sold it/them off to another dev) and the rest of the XF community benefits because the add-on is available for others to pick-up and fix/improve.
Add-ons for XF 2+ would be a perfect time to introduce this, if not beforehand.