We’d need to dive into the the details of Dutch taxation, it’s not clear to me it’s worse for investors than their current system (where before it was struck down by courts, you could be taxed on more than your actual gains, even with losses). It’s a flat rate rather than progressive so if you get market return (5-6%) you end with the same overall tax in the old and new system (tho old system was smoothed).
If you do beat the market then yeah you’ll be taxed more (but maybe those beating the market are also more likely to have large losses 
and revert to the mean long term)