At the point that a public company becomes a utility, why not nationalize it? The alternative being presented here is that private companies should be legislated and regulated into behaving in a particular way which is an incredibly inefficient way to manage a public good.

Honestly in practice I'm not sure there's much difference. A "nationalized" utility would still be regulated by legislation to behave "correctly" when operated by the executive branch. See e.g. USPS.

I think the difference between the USPS--which has laws preventing it from making a profit--and a US-based tech firm that profits by selling user data to advertisers would be a massive one.