> especially on unrealised gains.

Right, absolutely brilliant idea. You live prudently, save some money every month and invest it (stocks, bonds, whatever). Due to factors entirely outside your control like a stock market bubble or an interest rate drop, the $50k portfolio you built over 30 years is now worth $70k. Your unrealized $20k gain is taxed at 10% for easier math. You don't have $2k cash on hand and are forced to sell some of your portfolio to pay the tax.

Next year, there is a crash. You now have just $40k in assets. But there is a gradual recovery, and the year after it's back to $50k. You now owe another $1k. Sound good?