Instance Raids can be fun, sometimes. They are usually good for a run or two. But there has never EVER been an MMO in which Instanced Raids can maintain the level of fun necessarily to entertain a population for more than a few runs. i.e less than a week.
And can you name me one boss that is as good or as fun as lich king from wow or Twilight Ascendant Council in a open world because i never seen it done