I Mirror this sentiment.
The problem with comparing it to other ~70 Weapons is no other DD-job is forced to melee in an outdated weapon because it helps the job, all of them have gotten upgrades to those weapons in generally every field. There is not a single other comparison. This weapon is forced in our hands rather we like it or not. Unless they impose a Treasure Hunter Limit from Equipment of +1/+2 (I.E hands + Feet, so weapon is free) There is no other solution outside of upgrading it to make it useful enough. (note i said Solution: ignoring it is not a solution <_>)
If you're joining us now, I'm not asking for it to become god of weapons, Only that its more powerful than it is now. I.e Respectable. Any intelligent Thief who's not trying to play devil's advocate will tell you that they feel Thief's Knife needs an upgrade if we're going to be forced to continue to use it.
Truthfully, I have a few options)
1) Upgrade Thief's Knife. Via Synergy, or Trial of the Magians, or some other Quest (perhaps a tie in to the end of the THF AF quests). Make it difficult if you have too, but try not to make it involve camping a new NM or something similar. I.E if you're going to make it "Get xxx drop" have "xxx drop" drop from multiple enemies, Lots of them. Preferably not in dungeons.
2) Create a new level 90 Weapon that shares a name with Thief's Knife, allowing you to only hold/own one (is it possible?)
3) Impose a +TH Cap from gear to +1 or +2.
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I like options 1/2, Option 3 is a last resort.
Edit: As mentioned before, Zoraal Ja's Axe (Lvl70 Reward+10%) is the closest comparison i can think of when it comes to an outdated weapon being useful, But its not so much forced in their hands, more of preference.