0) Disclaimer: the following view is NOT a view of GM team, TMWT or something, it's just my private opinion.
I'd say...
1) I don't like completely unobtainable items, its waste of art and annoys players. However...
2) Many RPGs have tradition to honor players who has been there with them and took part in certain events. At the end of day it encourages people to take part in events - you can get uncommon rewards, etc. Events could very well be unique - and boast more or less unique rewards. Some "scripted" events/quests also been unique. Part of them eventually reused/extended/replayed. However, if you missed e.g. saving Jande, well, you can't easily see this part again. And, in the end, lore would generally remember only few certain heroes who did it for 1st time.
3) In RL it also happens like this: be in right place at right time - and get some unique artifact, huge discount or so. Miss that, and you'll never get second chance. But still many other things to try. Look, I can't buy some items I like - just because they're not manufactured anymore.
4) I'd say dropping ultra rare artifact by mere stupid terranite would be kinda silly.
5) This said, GMs do not generally create items on their own at random. Guess it takes quite a consensus across GM team and TMW team to do that. Which obviously raises bar, that's for sure. And so it could be rare/hard to assemble kind of thing.
Additionaly, it's more or less "badge" kind of item, not anyhow mandatory to conduct normal gameplay, do quests, etc. However if it's part of set I'd say making it somehow obtainable makes some point.
So I have an idea. How about making it a (very hard) quest? It also preferrably to feature hard boss fight, etc - with something like maybe "terranite king" or so, that maybe only appears sometimes, and takes a reasonable team to dispatch. This way it would be more defined than if/when GMs/TMWT manage to agree event, yet still big challenge that supports MMO part of game. Basically, some "relatively rare" raid can be an event on its own - taking no approval beyond meeting quest start requirement. Maybe it can (re)use Xakelbael-like mechanics to determine a winner, etc.
p.s. these are just views and ideas and there is one big nasty catch: it assumes someone could/would script that, assemble epic boss, etc . And ofc winning that have to be quite an achievement. Otherwise game would get swamped with this kind of thing. This said I dont really like TMW approach like "relatively easy boss, but very low drop rate". Guess "epic almost impossible boss, high drop rate" is more fun.