Everyone will sell their shares December 31st or earlier that month. After January 1st, they will buy the shares back.

Doubt that would work. I don't know how Holland does it, but one of two things would happen:

1. Wash sale rule, counts as not sold. Still taxed on unrealized gains.
2. No wash sale rule, it would become realized gains. Those are already taxed.

Either way, they still get their taxes. The whole reason they want to taxed unrealized gains is because if you buy and hold for 20 years, they're not getting tax revenue. Getting taxed on realized gains isn't a workaround for getting taxed on unrealized gains.

Obviously, this is assuming unrealized/realized are taxed at the same rate. If that's not the case, it would depend on the rates if it made sense to do.

The real question is if it's just stocks or they'll do it to other assets. Imagine sneaking in a 36% tax on real estate appreciation, forcing everyone to sell to afford it.