the downward pressure against the investor would naturally discourage the use of tax havens.
I would really prefer that you not apply "downward pressure against" me to stop CEOs from doing something completely legal that you dislike.
The intent is that there is nothing stopping you from investing in corporations that properly file their taxes, instead of spending countless man-hours planning how to very precisely allocate intellectual properties within a corporate structure to make their tax burdens as close to 0 as possible, through aggressive use of tax havens.
EG, They could just operate, and have IP assigned where it was actually invented-- and pay proper taxes-- and your capital gains would not get the special tax code---
OR
They can do the double irish bullshit, and their investors get the tax burden instead, depressing the market value of their stock, such that their tax avoidance has no real value to them.
Either way, the state gets its money, the value of the tax haven evaporates etc.
Taxes are not a bad thing. They pay for essential social services, and are one of the ways we get to have nice things as an advanced society. There are real negative consequences to wholesale tax avoidance, no matter how "Legal" the system claims it to be. This is why it should be discouraged. Special tax code for capital gains held by tax avoiding corporations seems a reasonable checkvalve.
As you yourself pointed out, the different tax structure of capital gains is intended to capture this "normal corporate profit taxation" value, such that ordinary investors are not double-taxed.
However, taxing the individual investors a small pittance for assets held in consumate tax avoiding entities, results in the EXACT SAME reduction in value. (EG, the elimination of the value added by the tax haven), so either way, the investor makes the same return. It just eliminates the value of the tax haven.