Key Reasons Taxing Unrealised Gains Is Problematic.
Liquidity Problems: Taxpayers may not have the cash to pay tax on gains they haven’t actually received. Both mitigation and remediation measures incur additional costs for taxpayers that are not captured as tax revenue to the government (for example, fees and interest on borrowing to cover tax liabilities, opportunity costs from changing investment strategies, the cost of maintaining extra cash reserves to cover unexpected tax bills)
Volatility Risk: Asset values can fall after being taxed, resulting in people paying tax on gains that later disappear (“phantom gains”).
Valuation Difficulties: Many assets, especially private businesses and real estate, are hard to value accurately every year, leading to disputes and administrative headaches.
Administrative Complexity: Annual assessments of unrealised gains create significant record-keeping and compliance burdens for both taxpayers and tax authorities.
Distorts Investment Decisions: May discourage investment in long-term or illiquid assets, harming economic growth and innovation.
Asset Price Distortion Near Tax Time: The approach of the tax assessment date can trigger artificial selling pressure, increased volatility, and price distortions, as investors adjust holdings to minimise their tax liability.
Fairness Concerns: Taxpayers view it as unfair to be taxed on “paper” gains that may never materialise, or to be forced to sell assets just to pay tax.
Double Taxation Risk: You might be taxed on the same gain both when it is unrealised and again when it is realised, unless the system works perfectly to prevent it.
Unpredictable and Negative Government Revenue: Mechanisms to prevent double taxation (such as cost base adjustments and tax refunds when asset values fall) can cause the government to lose revenue in years of market downturns, resulting in unpredictable or even negative tax receipts.
Rarely Used Globally: Almost all countries tax only realised gains; taxing unrealised gains is internationally uncommon and controversial.
Reduced Competitiveness: Increased compliance and cash flow burdens put local businesses at a disadvantage versus competitors in countries without such taxes.
For balance, key reasons why some people argue for tax on unrealised gains:
Anti-avoidance: Some argue taxing unrealised gains prevents indefinite deferral of tax (especially for very wealthy individuals).
Wealth inequality: Wealth taxes (not exactly the same, but related) can target asset-rich, income-poor individuals. Annual net wealth taxes do exist in some countries, but are different from capital gains taxes.
I'm not opposed to Taxing Unrealised Gains because it is evil or fundamentally wrong to do so. I'm opposed to them because taxation policy carries with it a list of pros and cons and there are ways of achieving the same objectives that have far better tradeoffs. And the taxing of unrealised gains has so many cons in comparison.
Good government is about choosing far better options when they are available so we don't have to pay the costs of worse decisions.