To summarize the current Dutch personal income system: besides income from salary and income from own business (these are taxed quite high), income from investments (stocks, passive investments, real estate excluding your first home) is taxed quite low. The amount is simply a percentage based on the value (as per the start of the year) of your investments.
So in the Dutch tax system there is no difference between realized and unrealized gain. As such it doesn't matter when you buy/sell your investments. It doesn't impact your tax burden. The effect you get is that everyone's wealth just slowly erodes away, just like with inflation (unless your yield outpaces that).
It is essentially a wealth tax system. But I wouldn't call it low: currently, 6.17% fictional yield x 32% tax rate = 2% wealth tax rate - it is at the high end among countries with a wealth tax (https://en.wikipedia.org/wiki/Wealth_tax)
One important thing the article omits is that there is threshold under which you don't pay anything in box 3. If you own less than 57.000 eur (or 114k for a family) you don't pay this tax.
That seems like a reasonable approach. That's much preferable to a tax on realized gains and a tax on unrealized gains. In the US when you buy a mutual fund you're already paying a "tax", for example, Fidelity eats 0.83% if you invest in their FSLVX mutual fund [1].
That's not a tax, that's the expense ratio, which is basically describing fees captured by the fund manager. Funds accessible to Dutch investors involve similar ERs. It's not an alternative.
It can be thought of the same way, but not from the perspective that's under discussion. As such it doesn't really add anything except a new perspective. Why are you introducing it, what does it add?
Calling the expense ratio a tax is like calling the labor cost of your car repair a tax. The expense ratio is what the fund manager is charging to cover their labor and expenses. It's not a tax on the transaction going to the government.
So in the Dutch tax system there is no difference between realized and unrealized gain. As such it doesn't matter when you buy/sell your investments. It doesn't impact your tax burden. The effect you get is that everyone's wealth just slowly erodes away, just like with inflation (unless your yield outpaces that).
But with this new law that all might change.