I see your point, but it seems to rest on the rest on the way we determine when capital gains are realized. Capital gains accumulate regularly just like other sources of income; in my mind the tax obligation is incurred at the time of the gain, just like the tax obligation on other income.

For practical reasons the state does not demand the tax until the accumulated gains have been realized by selling the asset. This makes sense for illiquid assets like stock in your private startup, but I don't think it should change the calculation of when the tax was incurred and how much. For liquid assets like shares in public companies, I think it would actually be perfectly reasonable to demand the tax on unrealized gains.