The destruction of the D actually goes back to the sixth season of
TNG. Ron Moore and Brannon Braga (mainly Moore, if I remember correctly) were intrigued by the section in the
TNG Technical Manual about the
Galaxy-class saucer making a planetary landing, to they pitched an idea for the sixth season finale (provisionally titled "All Good Things..." showing that writers always recycle stuff) that would have had the
Enterprise saucer crash on some sort of diplomatic mission. Saucer crash, fade to black...
But there was no way that the television production could have afforded the FX to make it work, so Moore and Braga shelved the idea. Then when they were invited to pitch ideas for the seventh
Star Trek film, crashing the
Enterprise saucer was one of the ideas they brought out since a film would have the budget to do it justice, and Berman agreed.
A new ship let the films have their own "hero ship" and visual identity, with
Generations as a transitional piece. But that's really an after-the-fact justification. In the moment, when
Generations was being plotted, the D's destruction was simply because it was something they could achieve on a movie budget that they never could have done on a television budget.
And I should have flipped over to Page 2 of the thread, because I'd have seen that
Dukhat had already covered this. Not trying to steal your thunder!
