If the above argument is correct, then determinism either entails that all of our beliefs are true, or that we shouldn't try to believe what is true. I assume most determinists would deny that we should try to believe what is true, given those options. But if they do that, then they undermine the entire basis of rational discussion and argumentation. So, they have no basis for saying anyone should believe determinism, or, indeed, accept any arguments for anything.

Rationality is about making choices for good reasons. Determinism is incompatible with making choices for reasons at all. The argument is pointing out that this doesn't just negate the legitimacy of moral judgment, or even prudential judgment, but of epistemic judgment as well. Determinism entails that all human beliefs are arational. This entails that all enterprises seeking to judge or affect people's beliefs are as well.

Determinism is incompatible with everything we know about ourselves as rational and moral beings. It's a wildly implausible view. There are also absolutely no good arguments for it. The fact that modern support for naturalism gets people to believe it anyway is comparably as embarrassing to the fact that followers of Parmenides got talked into believing that people can't move. Crappy methods lead to crazy results the followers of them are blind to.

We can walk. We can choose whether to walk or run. We can be irrational and immoral in our choice to walk to a drowning child instead of run. We can be irrational in any choice to believe in whatever crappy view forces us to deny any of these things. Determinism forces us to deny obvious facts about human nature, and it renders rational discussion and judgment impossible. It's a really stupid view. People who choose not to recognize this are wilfully blind to the obvious entailments of their view. It's important not to let that blindness spread to others.

Well, no advocate of any position believes the arguments against their view are correct. There's still value in explaining why their views are wrong so that others don't make the same mistake they did.

The argument demonstrates one way in which determinism is incompatible with standard assumptions of epistemology and moral responsibility. Also, "ought implies can" is about free will, no matter how much silly compatibilists want to pretend otherwise.

If you're opposed to large scale efforts to stop babies from dying needlessly, I think it's time to rethink your ethical framework. Large-scale social change is insanely difficult to bring about, and history has shown that it almost never has the results the people pushing for it expect. If you're okay with just letting people die for no good reason while you wait around for it, I seriously doubt you actually care about people at all. You're just a poser who likes to feel good about yourself without actually having to do anything.