The OP is making a (toungue-in-cheek) argument against corporate free speech. You said that corporate free speech rests upon a century of precedent and that without it, we couldn't have corporate immunity.
But "corporate immunity and corporate free speech come from the same legal doctrine" is a lot different from saying "you can't have corporate immunity without corporate free speech". The latter is what you said, but your argument supports the former. Are you sure I'm the one who is confused?
Well, I think that's far and away the most obvious interpretation of your second paragraph, but it's ambiguous enough that I'll give you the benefit of the doubt if you're saying that's not what you meant.
Your last sentence surprised me, though. Clean-slate thought experiments are useful to figure out whether you're doing something because it's the best way to do it, or just because of legacy. That goes for both code and law.
In particular, some of the other critics of the Citizens United decision, including ones who are a little more serious and a little less playful than the OP, are talking about a long-term campaign for a constitutional amendment to address this issue (I'm in particular talking about Larry Lessig and Change Congress, but there are others). Such efforts might be political longshots, but how do you engage in discussions with them if you don't want to consider all the ways the law could be radically different, or could have evolved entirely differently?
Human political freedom in the US is something I'm interested in, and I'm not actually interested in wandering off the topic. I would hope someone engaging me in discussion on the issue wouldn't try to pull off into other directions - that would be disengaging.
But "corporate immunity and corporate free speech come from the same legal doctrine" is a lot different from saying "you can't have corporate immunity without corporate free speech". The latter is what you said, but your argument supports the former. Are you sure I'm the one who is confused?