When businesses ask you to contact their help-desk via WhatsApp, it’s a utility. When people call and message friends, family, and colleagues almost exclusively on WhatsApp or Messenger, it’s a utility.

Except...no, it's not. That's an extremely naive understanding of what a "public utility" is. A public utility is not defined by how many people use something. Public utilities are essential services that typically operate on economies of scale. That is to say services without realistic replacement and which have large upfront creation and maintenance costs and which only make sense to provide access to a large number of people. You can't replace electricity with some alternate source of power. It's electricity. Same for water. They're fundamental services that are required for other services to exist. Without electricity you don't have phone or internet. Without water you can't have sewer systems or indoor plumbing.

WhatsApp, by comparison, is trivially easy to replace. A business chooses to use WhatsApp for customer service. They could just as easily setup a Discord server or just establish an 800 number for you to call. They have immediate drop-in replacements. Arguing otherwise is sort of like arguing that Coke should be considered a public utility because a business serves Coke products. They don't have to serve Coke. They could serve Pepsi. Or anything else.

Also, your reasoning is kind of skewed, because in order to even use something like WhatsApp, you need other, already existing services. Namely internet access. It makes literally no sense to say "WhatsApp should be a utility" without first arguing that "internet access for all individuals at a national level should be a public utility." Which I would personally argue is something that does qualify as a utility, far more than any specific social media services or app, and the fact that it isn't is a huge problem for the United States.

Godwin’s Law People preaching [insert terrible belief] on a government platform would be removed and charged for hate speech just as much as they would be if preaching these things in public spaces.

Oh, okay, "Godwin's Law" is it? Cool. Here's an actual law. Like a literal piece of legislation that exists: it's called the First Amendment. I don't know if you're just speaking from a non-American context, or just don't know how "freedom of speech" is codified into law in the United States. Maybe you're a kid or something and just haven't learned that in school yet. But freedom of speech in public places is universally protected under the constitution. Like, there are still public Klan rallies in certain parts of the country. This is what allows those to happen. If the United States government maintained its own social media service, it would functionally not have the power to moderate any content that was not explicitly illegal. Bigotry and hate speech are not illegal under the constitution.