We need only to look to journals to see counter-examples to your proposition that "If a paper deserves a serious comment, I think the proper format for that is

We need only to look to journals to see counter-examples to your proposition that "If a paper deserves a serious comment, I think the proper format for that is another paper." For example, a journal might have a "Letters to the Editor" section for people to voice a serious comment, even though such a letter is not "another paper".