But given the cash side is unenforcable because of this property of fungibility, it's more like casino chips where you pay taxes on them when you cash them in?
I'm not sure whether the IRS would buy that argument. If you won $1M (in chips) at a poker tournament I doubt the IRS would let you report your winnings across 10 separate years to reduce your marginal rate. The tax is on income, which is earned when you won the tournament, not when you cash out the chips.