I have to agree with you about Mary Sidney, but that does not mean there’s no merit in fairly examining the claims of other candidates.
The clincher for me is that many of Shakespeare's contemporaries, close friends, and coworkers, including Ben Jonson, didn't say a peep about any ghostwriting and indeed stated repeatedly that Shakespeare was the author. The preface to the Folio, correspondence, and published works acknowledged his authorship. Contemporary evidence is to be favored over "cipher" analysis, speculation, and theories about motive 400 years after the fact. A conspiracy that involved all of Shakespeare's writer friends and his publishers seems highly unlikely.
I think most of the original objection arose because people thought that Shakespeare, "merely" a middle class, hardworking man and a player, could not have had the education to write about his subjects. People today don't realize just how good a typical 16th c. education was and how well-read anybody in literate circles in England had to be.