When a for-profit corporation exercises free speech, it is actually the speech of its shareholders. Therefore, in the case of corporate speech it's still a group of individuals talking. The only thing you could do is put an individual limit on spending, which is then translated into a limit on corporate speech by summing up the limits of the shareholders, with foreign shareholders contributing zero, and domestic ones seeing their limit divided over their shares.
> When a for-profit corporation exercises free speech, it is actually the speech of its shareholders.
No, it's the speech of the management. The shareholders only have indirect control. The only power they generally have is to replace board members. And sometimes they don't even have that. Google, for example, is completely controlled by Larry and Sergey. Facebook is completely controlled by Zuck.
Personally, I would have had no problem with CU if it had remained the narrow decision that John Roberts originally wanted it to be. Clearly, if the First Amendment protects Michael Moore's right to make and show a politically charged film close to an election then it protects Citizens United's right to do the same thing.
The problem came when this narrow decision was extended to a broader one that included for-profit corporations and labor unions. (For-profit) corporations and labor unions are legally "persons" in that they can act as legal entities in their own right (no pun intended) independent of any individual human. And they ultimately consist of groups of humans. But they are not humans. They are human constructs. They are technology. And treating them as if they were humans, entitled to human rights, leads to problems.
So personally, I would advocate retract CU protections (and Hobby-lobby protections for that matter) from for-profit corporations.
I'd also re-instate the historical lower limits on direct contributions to political campaigns.