The strength of a point is not changed a whit by where it is written.

Suppose you are responding to reviews on a paper and you are justifying using method X for an analysis rather than more common method Y. The reviewer would like you to use the more common method X, but let’s say that in your situation, application of Y would be inappropriate and you’d like to make this point.

Traditionally, you’d just make this point in your response. You’d write a few paragraphs explaining it, and the action editor would have to weigh what you say. But we can now apply every one of your critiques of citing a blog post to your response to the reviewers! Your response certainly isn’t primary literature; they are not “final product[s] that we can consider as having the same status as a manuscript;” and, they are highly subjective.

Now suppose that you learned about this fact about the problem with applying method X in your scenario from a well-known statistician’s blog. This statistician lays out the reasoning, which of course flows from his theoretical knowledge of the methods, but is not made explicit in any published papers. Why would you not cite that blog post? You absolutely should, because:

• the statistician gave you the idea (credit where credit is due),
• the statistician probably explains it more clearly than you (it is their job, after all, and this means you can make your response less verbose),
• the blog post offers a place for the reviewers/editor to query if they have questions (in the discussion of the blog post or with an email to the statistician), and
• the point is as strong regardless of where it is written (in your review, on a blog, or in a peer-reviewed paper).

The belief that all responses must be themselves vetted by peer-review is quite extreme. If you believe that a response can make points that are not found in any specific published paper, then it wouldn’t make any sense to avoid citing blogs. Authors would just make the same points, without giving the intellectual originators of those points credit.

I suppose you could agree to all this and still say “but editors won’t like it, so you shouldn’t do it” (your points 1 and 4). I guess maybe I’d even agree that peer review is driven by irrational biases like the one you describe. But if we wish to change silly norms, then someone has to actually go about behaving in a way contrary to those norms.