Is anyone's 'last' work, actually the last?

In class on Friday we were talking about Shakespeare's last play, The Tempest. But is this really his last play or piece of writing? Or is anyone's last paper their last, Plato's Laws, or Carrol's The Hunting of the Snark?
If everything is a displacement into something else, wouldn't Shakespeare's last work actually be a beginning into a web work of Shakespeare or Shakesperian influenced writing? So by saying it is his last work, wouldn't you also be saying that his work isn't timeless and ceases to have connections with another piece of writing?
I think one could argue that by the direct or indirect subjection of The Tempest that in a sense Shakespeare's last work just plain isn't. Through the collaberation of Shakespeare and whomever decides to use him as a basis or idea starter, wouldn't Shakespeare take on the title of a type of co-author? What I am trying to say is that since The Tempest Shakespeare has lent a hand from good writing to bad and everything in between.
By looking at his 'last' work this way would be a claim that The Tempest is really the beginning. After something has been written it isn't finished or complete, but has just started its perilous journey with no end in-sight.
The following is a small excerpt of how I came to my conclusion:
I think an important thing to note here is that on citations. If someone cites a work, in either works cited or works consulted, isn't the author in essence giving credit to that author? If they are giving credit, they must have felt there was some impact that could possible be seen by readers/critics as not independent. If the author feels that his work was created not only by his own ideas,but by building off of previous ones, wouldn't we consider these 'citations of impact' as actually co-authorship?