There's a difference between "x is not negative" and "x is positive" (¬(x < 0) ⇏ (x > 0)). Why shouldn't a similar subtlety (or a larger one) exist in prosaic language?
Yes, and 2 is not negative, and 3 is not negative, and 4 is not negative, etc.
That is the point. This style of communication is indirect and ambiguous. This is just negative followed by negative followed by negative, etc.
Just say what you mean. In the affirmative. Overuse of negatives is the functional equivalent of "spaghetti code" in written communication. Not easy to follow.
Anyway, some readers missed the point of the comment. It is not every day that one sees so many negatives in one sentence. Most however got the point, and the commenter who crafted a version of the sentence with even more negatives I thought was hilarious.
“not unreasonable” expands to “You could provide reasons that seem valid, even if I don't agree that they support your conclusion.”
“reasonable” expands to something more like “The reasons you have provided support your conclusion.”
“reasonable” can work in this case, but it doesn't state as clearly that the speaker disagrees with your conclusions.
In a more general sense the “not un-” pattern is a marker for something that is qualitatively similar to the corresponding simple positive attribute (e.g. “reasonable” or “popular”) but not to the extent of the category of things fitting that simple positive attribute. That is, category “reasonable” is a strict subset of category “not unreasonable”.
The point of the comment was not questioning the meaning of the sentence. The phrase "not unreasonable" is quite common. With some effort, we can decpher the meaning. The point was that there are other ways to express that meaning, using fewer negatives.
For example, something like this:
"I don't consider it unreasonable to believe that supply chains will survive to 2030, so please, don't feel attacked by my beliefs."