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.
[1] https://fundresearch.fidelity.com/mutual-funds/summary/31612...