All the other items are drops from mobs, TibiaCoins on the other hand, are a parallell currency that have value outside the gameworld.
With that being said, Tibia Coins are the only asset that in fact will always have value.
If you intend to 'profit' through any item, say, Demonic Essences, on the next update more mobs will drop demonic essences or the drop rate has changed. Their value will change accordingly, so unless if you have inside information about what CipSoft intends to do on future updates, any item is mostly gambling.
Whereas Tibia Coins have value no matter what.
You can change a lot ingame but Tibia Coins still have value and it's far easier to predict values than any other item.
If CipSoft decides to stop its operations tomorrow, in theory, what they'd do is 'but TC back at a very low value', they'd only buy tibia coins back, not gold, not items, not rares, nothing. Only TCs. That's usually how MMORPGs that have died operate.
It is, undoubtly the safest of items, in my opinion.