What it really comes down to is that from a marketing standpoint there is excellant reason to make a Star Destroyer. From the perspective of gamers a Star Destroyer could be insanely fun. The only reasons not to have one has nothing to do with anything FFG should be prioritizing. They worry about making a fun profitable game. If you can argue that Destroyers wouldn't be fun go ahead. But we won't know that until the Tantive hits. If you think aStar Destoyer won't make money you can argue that, but I would say you've missed what makes this game so great. It is only somewhat from the mechanics. Most of it is in the minis and the Star Wars name. And nothing defines the Empire more than a Star Destroyer. Not the Tie Fighters. Not the Death Star. And not Darth Vader. The first you see is that gleaming deathly triangle of doom. And it says everything you need to know about the empire. That's why the game should have a Star Destroyer.
your throwing assumptions around as if they were facts.
for me for example the game is great because the game mechanics and the minis reflect the style and flair of the Star Wars theme in a great way and aren't just an excuse to sell yet another Star Wars themed game and scale is an important part of that equation. I don't presume that everyone has to see it that way, but also it seems that not everyone is sharing your view.
as for money, sure a ISD would sell, will it bring more money than keeping the style and scale - I honestly don't know. But again assuming it will be the best financial strategy is mostly a guess.