As a general rule, when engaging with thoughtful people, if your presumption is that it's obvious you are correct and a phrase like "big deal" enters ones repertoire, it's probably a sign you are not arguing the same point, since if it was so obvious then thoughtful people would have come to the same conclusion.
One reason this issue is worthy of debate is because it is novel - we have not been a position before where there are a few global scale 'public squares' under centralized corporate control, through which it seems inevitable the whole world's discourse will flow through in some form or another. Nevermind the fact that these corporations' business models are directly tied to influencing human behavior on those platforms - the incentives align in a direction that seems incredibly harmful both in theory and practice.
As society evolves and advances, particularly an advanced society like ours with many exponentially advancing elements, new emergent phenomenona appear. We ought to not appeal to tradition to guide us in all cases: we could be dealing with something genuinely new that our generation needs to address directly, because our ancestors could not foresee it, even though they tried to create generalized systems. I would argue that is the case now, much like it was the case in the response to large changes in society like the industrial revolution and the creation of nuclear weapons. In fact, your mention of monopolies is an exact example of this: we created this concept in a legal sense, in response to unexpected damages that come from having monopolistic actors. If anything, leveraging the argument of monopolies existing or not in your argument is a tacit concession that such structures can be created and have moral weight in determining how we should act.
In our era, arguing society needs to create new legal and policy structures to ensure social media and these global public squares best serve us all seems reasonable to propose. We seem less likely to do that insofar as people reject the idea that such a discussion is ridiculous, because we already have all the existing mental models and tools we need to address the problems they bring.