If you wanted to tax the wealth.. have a wealth tax? It's a thing in countries including Norway and Switzerland, and similar things exist in others (e.g. Italy taxes assets in some weird non linear way).
The idea that a gain isn't a gain until the asset is sold is nonsense. That'd literally be like saying that Bill Gates or Warren Buffett never made a dime on any of their stock that they are giving the charity. It's a ludicrous conception of capital gains, but it's exactly the one we use.
Which is the point, really, cause nobody cares about the small person who got a 1000% return on 50$ of Bitcoin, we care about massive assets owned by a tiny minority who is able to dodge capital gains tax even if their asset growth is only 4% per year.
A capital gains tax taxes the $2,000,000 unrealized -- but eventually effective -- gain.
A wealth tax taxes the $3,000,000 estimated net worth.
There is a real distinction. A real reason to incentivize productive capital, and that is you get to keep what you've earned. The point I'm trying to make is only that people should pay taxes, and an unrealized capital gain from literally 40 years ago isn't some unknowable investment outcome... it's a capital gain.
I think micro taxation on all transactions (including at ATMs) would work better. Easier to explain, harder to avoid since you tax both sides of each transaction.