> But you haven't gained...
Yes, you have. You have an asset of greater value which you can leverage in a number of ways without liquidating it and "realizing" the gains. That's a real gain, with real value.
> you could be taxed over and over again
Only if you make new unrealized gains.
> and if the stick drops or hits zero then what?
Then you have a negative unrealized gain, or, equivalently, an unrealized loss. If you are taxing unrealized gains instead of taxing gains when realized, then the natural assumption would be, just as is done with taxing gains at realization, that negative unrealized gains are either offset against current income or against future unrealized gains, and so effectively create (considered on their own) negative (current or future) taxes. The simplest form of this is to offset only against future gains, by the simple mechanism that when gains are recognized for tax purposes, they adjust the basis value of the asset, and when unrealized losses occur, they don't effect the basis value at all, so you don't have a taxable unrealized gain again until the market value exceeds the basis value established at the prior peak.
More complex versions would allow you to offset some or all of the unrealized loss from the prior basis value against current income of other forms, but the amount of that offset would reduce the basis value of the asset.