> Suppose the Big Tech companies are far-right instead of left.
Wait, when did corporate capitalism become a left-wing position?
What you really mean is "supposed the Big Tech companies became far-right instead of centrist neoliberal capitalists".
> In this scenario people wouldn’t be saying “freedom of speech is freedom from compelled speech; you can say what you want but you just need to create your own platform / internet; etc.” T
Yeah, just projecting a story of how people would act in a counterfactual in order to base an argument on it is problematic because its obviously quite easy to just assert whatever behavior would best suit the conclusion you want to draw.
One could equally easily assume that left-wing opponents of the action you suggest would, instead of focussing on state regulation (presumably, if industry was that much more tilted against the left than it is now, through the influence of industrial money the government would be even more tilted against the left than it is now, which would make the government unhelpful in any case), the left would instead organize outside corporate communication channels (as it has in the past) for boycotts, demonstrations, and mobilization for both the interests that were being targeted and against the firms that were trying to block them. Of course, in that case, the argument you are trying to build on top of your story about how the left would behave falls apart.