Your quote highlights why your proposition is a bad idea; it’s semantically incorrect. __text__ is strongly emphasized due to being surrounded by double underscores. The <i> tag does not mean strong emphasis/importance.
Not about ‘bold’ or ‘italics’, it’s about emphasis and importance. In which case you could only argue for _ as emphasis and * as importance. This would be incompatible with having double asterisks or double underscores as means of indicating importance, resulting in massive backwards compatibility issues.
Not to mention that your proposal from the beginning already breaks many, many existing documents and implementations. (Everyone’s single asterisk wrapped text would be changed from being in <em> tags to being in <strong> tags, and vice versa for double underscore wrapped text.)