What is the significance of the play within Shakespeare's play A Midsummer Night's Dream, especially with respect to parallel subplots?

One of Shakespeare's central
points in A Midsummer Night's Dream is to show the
distinction between illusion and reality. He even shows
that the happiness and success of life hinges upon
illusion. The mechanicals' play within a
play is central to Shakespeare's
illusion theme because all of the mechanicals prove to have
their own illusions about their abilities to perform a
grand play. In addition, their performance of the play in society serves to link
illusions with everyday life, to link illusions with reality. However, it's important to
note that the plot of the play within the play is actually
the exact antithesis of the plot within A
Midsummer Night's Dream. Shakespeare's A Midsummer Night's
Dream ends happily with all of the couples in the play either uniting or
reuniting, but the play within the play is actually a
tragedy. Pyramus and Thisbe both die by suicide. The
antithetical plot of the play within the play shows us that
while illusions may be antithetical to reality, the
two can still be
united.

During their performance of the
play, the mechanicals are certainly subjected to a great deal of criticism
and jesting, which they most likely can hear. For example, Hippolyta
calls the moon, which Starveling represents with a lantern, a bush, and his dog, tedious
and complains, "I am weary of this moon. Would he would change!" (V.i.250-51). However,
when the play is done, Theseus ultimately praises their
performance, saying, "And so it is, truly [a fine tragedy]; and very
notably discharged" (355). And this line is the last we hear of the play within the
play. Most likely, the players accepted their praise at face value and never became
disillusioned about their abilities as performers. They remain clinging to
their illusions, showing us that, even though antithetical,
illusions and reality can unite.