You can think yourself out of anything. These thoughts are probably also very reasonable. So you’d list the reasons why not to do it, for example why not to start a blog / newsletter. One reason is that you could be completely unqualified to share any expertise on the matter (any and all matters). Another reason could be that you’re prone to rambling. Yet another reason could be that you could potentially create something so horrendously bad or factually incorrect that it destroys your reputation and brand. These are good reason for not starting.