The proofs have been melted both in '79 - '80 and again in '08 - '11.
The first time there were a lot more of these sets around to melt so more got melted. Probably an even bigger off-take of the supply though is degradation. The coins get corroded or are stored improperly.
They are hardly uncommon but we may be taking them for granted more than we should. I'd guess more than one third survive.
Time don't fly, it bounds and leaps.