"If you wish to be persuasive, you must be diplomatic."
I don't think that's always true. Learned readers are not persuaded to accept conclusions of arguments based on diplomacy. Learned readers are persuaded to accept conclusions of an argument based on the veracity of premises in that argument, the level of inference of those premises and, finally, whether the form of argument is valid. If you have solid premises inferring what they are proposed to infer, and if those premises are composed in a known valid logical form of argument then diplomatically presented or not the learned reader will be persuaded.
That said, on forums like this worrying about persuading is a waste of time. I think at most we can hope to share good information and learn along they way. Sometimes what we learn is not what folks think was there to be learned, but it is learning nevertheless.