Last thing anyone wants is an auto-avoidance from an airbus up top because someone didn't bother getting their transponder checked out.
Did investigation actually show he hadn't had his RADs done?
Regardless, this is not a small amount of creep; such an event is a major malfunction that can happen a day after it's been signed off.
At the same time, if I fly today and ATC reports no issues. But if the 2 years expires tomorrow and I can't get it done in time*, leaving it off, as the law says you should, would be counter productive to safety.
* yes, you should have planned ahead, that is not the point.