I think that one way Shakespeare holds the interest through the use of thematic development in the Prologue is by simply summarizing the drama for the audience. Through the Prologue, the audience absolutely knows how this is going to turn out and what will happen. Yet, it's the development of the themes of love and subjectivity within external reality that holds the audience's interest. The idea of "star- crossed lovers" helps to hold the fascination of the audience with its assertion that what is experienced internally is not shared externally. The "grudge to mutiny" element helps to hold the audience's interest in showing how such a collision between subjective and external will unfold. For Shakespeare, the Prologue is used to outline how the drama's themes will unfold. Even though the audience knows what will happen, interest is maintained to see how these themes will reveal themselves. In the traditional sense, love was seen as the force of redemption. One way in which Shakespeare uses the Prologue the hold the reader's interest is to use it as the theme of the drama. The "mend" that ends the Prologue might serve as a way for Shakespeare to openly claim that while the theme of love will not end happily, it will be displayed in such a manner that one can better understand it. In doing so, interest is piqued using themes that the reader can appreciate and anticipate in the course of the drama. The Prologue establishes this at the outset.
This is a great question, as really any audience should ask themselves what the point is of watching a play where the ending is made clear in the very first scene. Note what the Chorus tells the audience as the play opens:
From forth the fatal loins of these two foes
A pair of star=crossed lovers take their life
Whose misadventured piteous overthrows
Doth with their death bury their parents' strife.
After hearing this, why listen on? However, Shakespeare manages to use techniques that have been adopted by playwrights and screenwriters throughout the centuries to engage and manipulate the audience into wanting to stay and watch what happens. Even though the audience is told in the opening scene how the play will end, Shakespeare deliberately raises the hopes of his audience by tantalising them with the possibility that it might actually work out for Romeo and Juliet, and clearly makes the audience feel sympathy for these two tragic characters. The audience are made to actively want Romeo and Juliet to take on fate and win, and it is this hope that keeps them entranced by the action that takes place before them. This is the technique that Shakespeare uses to maintain his audience's interest, even though the audience knows how the play will end.
First, this question seems to assume that the standard case is that the ending of a play is unknown by an audience and the interest is normally maintained by suspense. In Shakespeare's period, this was not necessarily true. In all the classical models, on which much of Renaissance drama was based, the audience was already familiar with the plots of the plays. Many of Shakespeare's plots, including that of Romeo and Juliet, were not original, but taken from existing stories which may have been familiar to his audience. Thus what held audiences was not suspense in the sense of finding out what would happen at the end, but in part the experience of watching how the nature of the characters impelled them to their tragic denouement and how thecharacter's choices determined their fates. Finally, as well as the story of the lovers, we also get to watch unfold how the politics of rivalry between great houses does so much harm to the city, and bears a lesson in practical morality for England, as resolved in the last scene:
See what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love!
I think that the opening characterization of Romeo and Juliet as "star- crossed lovers" does much to enhance the characterization in the Prologue. Understanding that the protagonists are "fated" to a great extent enables our interest to be piqued. With stating that the death of both young lovers will "bury their parents' strife," Shakespeare has been able to clearly establish that the characterizations offered will be fundamentally different than what the current setting offers. In this, the characterization offered is more of what can be as opposed to what is. This is an element in which interest is maintained. The audience becomes engaged in seeing how the characterizations offered will be so fundamentally different than the setting in which they live. The audience realizes that the end will be tragic. Yet, there is interest in seeing how this will come about. There is something uniquely distinctive about characters who wish to see more of what can be in a condition that is bound only to display what is. In this, there is challenge and difficulty. Through this, there is interest maintained and this is how characterization in the Prologue is able to sustain audience interest even though the ending is clearly evident.