“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”.
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."
