Assume, for the purposes of the argument, that corporations aren’t people. I don’t think this implies that the Citizens United decision is necessarily wrong. I think it is obvious that even if you accept fully that corporations aren’t people, people are people, and retain their First Amendment rights of speech and association. The obvious conclusion is that people retain the right to use a legal fiction to represent themselves as a matter of free speech and association.
Put another way, free speech and association is the foundation of any kind of corporate entity. If people were restricted from speaking and meeting with each other, and from expressing their point of view, certainly there would be no corporations and no corporate speech. But would this be desirable?