If you change the Death Star into coruscant at the end you also need to change the admiral Ackbar breefing. And although the audience was told that the Death Star is not yet operational, it would be odd to not see it by the end of the movie as it is set as a major plot in the begining of the movie. The whole Emperor's plot is that the rebels must think that the Death Star is not operational, but it is.

What would be the emperor's great plot if the end was set on coruscant? I'm sure we could find something, but not without changing the whole movie and redubbing a lot of characters... I don't think it could work.