Yes, it will be discontinued.
It's just another money making scheme jagex has come up with.
They will never be as rare as santas, partyhats etc. Never. Back when the old rares were introduced, no one knew about hoarding. No one hoarded partyhats, santas and other rares, they simply thought of it as junk.
Now people are hoarding fish masks, thinking it could become like those rares. But everyone that thinks so forgets to rule out, there was no hoarding back in the day. And less were in the game. Within the first 2 hours of its release, there were people showing 1,000 fish masks. I'd say there would be 100-200k by the time it's discontinued, possibly more when people buys sof spins. Overall, there will be enough for everyone in the game. Unless the game takes a huge boost where its players increases to 300k+ simultaneously, it won't be worth much.