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).

But with this new law that all might change.