Perhaps. But with HN you cover the more blatant loopholes and counter-arguments quickly, and can then think around these arguments to discern truth.

Not every article deserves critical thought. HN enables you to filter out the ones that do.

Only in the same sense that calculators are a tragedy, when the simple takes less effort, there is more effort left for the complex. The analogy of an externalised group brain is actually a pretty good one, and one may contribute to said brain as much as one benefits from it, to boot.

Maybe this is my own prejudice, but I think there's a big difference between practicing basic math skills constantly, and exercising critical thinking constantly.

Sure there's quite a big difference, but it's the same basic idea, a concept which through independent enquiry may only occur to you after period of time x has elapsed instead is presented to you in a shorter period of time than period x. That doesn't necessarily mean you don't critically analyse the presented concept, or that you don't analyse other aspects of the problem space merely because you have neatly parcelled answer handed to you, it's just a single aspect which you did not independently need to construct.