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I'm not especially bothered about Aunt Flo, but I think the issue with CGT is it really only kicks in when you have realisations. If you are sufficiently wealthy you can indefinitely borrow against your asset base (or harvest its dividend income) to support your lifestyle and never trigger CGT.
Taxing unrealised gains is a hard problem (and raises the obvious questions of how they are valued and how you treat unrealised losses).
Some, yes, I don’t think 100% tax would work, but the thing that separates rich and comfortable from those on the treadmill is assets, not income, and CGT is the niftiest way to tax assets without worrying about kicking Aunt Flo out of her Kensington townhouse.