They made a thread about it a while ago as I remember. If they ever did bring them back it would probably be as super rare treasure trail rewards. Don't think it would ruin the economy. Prices would stabilise over time, just like every other item.
Jagex stated themselves MANY times they didnt intend for partyhats to be hoarded, insanely valuable and did not think of the reprecussions at the time of making the items tradeable. Also, when rares were introduced, no one had anywhere remotely close to a max cash stack and at the time of the drop only a handful of players had a single, yes just a single 99. Think of it as a lucky mistake. At one point they considered removing rares, in a 2003 news post.
OT: I want my rares asap.
"Rares, however, are what you described as a lucky mistake which added on to the uniqueness of Runescape."

