> The unrealized value is notional, not actual.
No, its an actual thing, measurable by some mechanism. Otherwise, this would be a non-discussion, as taxing it would be impossible, not a possible thing that we can argue about the merits of.
> The notional value is often not remotely realizable.
Whether it is or is not immediately realizable is immaterial to the desirability of taxing it; it may be material to designing the forms of taxation that should be acceptable. E.g., if the difficulty of realizing the value is, across the tax base, likely to making collecting the tax in cash or equivalents difficult, it would argue for permitting a fallback option for the tax to be collected in-kind, e.g., by the taxing jurisdiction acquiring a proportional interest in the asset equal to the share of the value of the asset represented by the taxes not paid by other means.
> A tax on unrealized gains is in effect a way of laundering a steep tax rate so that it looks “small” and therefore reasonable to the unsophisticated.
If you allow carry forwarded losses, even just by the simple method of adjusting basis values, and include taxes on realized gains (and carry forward, offsetting against current income with perhaps a negative net, etc., for realized losses), then taxing unrealized gains is identical to taxing realized gains if the gains are eventually realized, but simply avoids the ability to find maneuvers to benefit from leveraging the value of the asset without paying taxes by avoiding realization. It doesn't make a "steep" tax rate look small, it makes the tax rate look like exactly what it actually is, unlike taxing only realized gains, which makes an effectively non-existent tax on capital gains look like something more, when people can benefit from assets without realizing the gains.