Several of Shakespeare's plays are collaborative efforts
We know several of his plays were collaborative efforts. It is possible many more were.
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Several Shakespeare plays were collaborative efforts with other authors. It is, therefore, possible that many more are collaborations than previously thought.
Collaborating on plays was common in Elizabethan England. In the face of evidence, Shakespeare’s works were likely more of a collaborative effort than previously believed. Around half of Shakespeare’s final 10 plays were co-authored with another author. He wrote ‘Two Noble Kingsmen’ with the help of John Fletcher. It is also now clear that some of his earlier works were also joint efforts between multiple authors. We know that some of the most beautiful sections of Shakespeare’s ‘All Is True’ were written by John Fletcher, who also wrote parts of ‘Henry VIII’. Ben Jonson may have collaborated on Julius Caesar. Christopher Marlowe, Shakespeare's greatest rival, also likely contributed to the Henry VI trilogy. The development of AI and machine learning is allowing us to detect patterns in Shakespeare’s works that indicates the existence of multiple authors in some of his seminal texts. For example, the writing patterns in ‘Edward III’ have led scholars to believe that the play was the result of multiple authors working together, with Thomas Kyd almost certainly working on the play as well. This technology has also confirmed that ‘Sir Thomas More’ was likely a collaboration between Shakespeare and Anthony Munday, among others. It also revealed that Shakespeare may have only written around 20% of the text for ‘Henry VI Part I’. Several different writing styles are visible in the play, suggesting that there was a large team involved in the project. In the case of ‘Titus Andronicus’, Shakespeare and George Peel appear to have worked on the play together. Unlike many of the other collaborations, each author wasn’t responsible for their own part. Instead, the pair worked on the same parts together. Thomas Middleton appears to have had a hand in ‘Macbeth’, and likely had some input in ‘All’s Well That Ends Well’.  As technology and machine learning technology develop, and we become more adept at spotting linguistic patterns, we will likely discover that other authors had a hand in almost all of Shakespeare’s plays. This dramatically changes the image of the single Bard churning out great work after great work. The credit should instead go to the team of playwrights.
Shakespeare undoubtedly collaborated on many of his plays. This is indisputable. But it does not have much of a bearing on his legacy. Thomas Edison, for example, collaborated with many inventors on some of his most prominent inventions, but it does not undermine or belittle his achievements. Shakespeare was undoubtedly the head of the collaborative process. His name is on the texts and he wrote the bulwark of the prose. Therefore, he should receive the most credit for the literary works. The advent of modern technology, which has allowed scholars to determine exactly which lines were written by Shakespeare himself and which were the work of his collaborators, has only served to further illustrate the Bard’s literary genius. The lines written by Shakespeare himself are the ones that are often the most striking and have the greatest impact. For example, in ‘Henry VI Part I’, we know that Shakespeare wrote as little as 20% of the play, but the segments he did write (Act 2 scene 4, Act 4 scenes 2-5 and the opening of scene 7) are the most powerful and inspiring sections of the play. 
[P1] We know Shakespeare collaborated with other authors on some of his plays. [P2] Modern technology is revealing that he collaborated much more often than previously thought. [P3] Therefore, it is reasonable to conclude in the future we will discover more evidence of collaboration.
Rejecting the premises
All of this is true, but irrelevant for determining Shakespeare's legacy.