Just as Romeo was about to drink the poison, Juliet wakes up and stops him! Wouldn't that be a great ending?and the triumph of true love? Why didn't Shakespeare write the ending this way??
Well, because the story is supposed to be about "true love" the kind of love that never dies, and that required the death of the couple in the
early stage of their
forbidden love.
Let's assume they do survive that last scene and escape together : Juliet-coming from a wealthy family- will be dissatisfied with the inferior way of living, but- being a lady- won't express it to her husband. Romeo would feel it and will try to compensate his feeling of inferiority and failure by either mistreating Juliet , or cheating on her, in a way to prove his masculinity. Hence, fights and miserable life.
Adding to that the resentment they feel for leaving their family,parents, friends ,past simple life .. we get another sad ending that may be the indirect suicide of Romeo and Juliet, either by overdose or a car accident caused by the depression they fell in because of loneliness and the failure of what once was a "true love" that turned into a "reality".
Their love was so great because they were young, it was still in its begining,and it was forbidden.
To make such strong love immortal, shakespeare had to kill the mortals before they kill it.