I think we'd want that tax income because a dollar today is more valuable than a dollar tomorrow - making sure we increase cash flow in the short term and stabilize expected cash flow allows us to more accurately set the budget.
> You may never have future gains to match the loss.
C'est la vie. That's how Capital Loss deductions, Electric Vehicle credits and other non-refundable tax credits currently work: individuals may not have enough upside to maximize their benefits, That's just how the system works as it is impossible to balance everyone's competing scenarios. If you are eligible for a $7500 EV credit but only paid $3000 in taxes, you'll only get that $3000 back, and not the full $7500.
Why would we want a system that taxes people on an unrealized gain and then (maybe) gives it back to them over coming years?