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.
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)
They are replacing it with a much worse and untested economic policy - taxing unrealized capital gains every year. Not a big deal for relatively stable assets (real estate etc), but can explode in your face if you're into any risky volatile stuff (stocks, options, crypto) - they can crash next year, but your tax bill won't. Lack of liquidity can get you as well - you may have huge gains on paper, but for various reasons unable to sell in a reasonable timeframe and come up with equally huge amount of cash for the tax office - we're talking probably 30-50% tax here vs 2% under the old system. Double taxation if you have US passport - you're going to have to please both tax systems or pay double the tax.
The outcome I'd guess would be an exodus of the rich / upper middle class, and then they either scrap it or tighten further with exit taxes. Oh and they're also scrapping the coveted "30% ruling" for expats. Probably can forget about ever being able to FIRE in Netherlands.