You know what they say, "Hindsight is always 20/20."
It's pretty easy to say that something seemed obvious 4 years after it happened, but while these things are happening, there's definitely a lot of room for uncertainty.
I don't think anyone is to blame for not predicting Dark Pit as his own character, or for not expecting/wanting DHD. At the time, we really couldn't be sure about anything, as the Smash 4 speculation period was a roller-coaster of surprises for everyone involved.
2.5 whole years of speculation really does give birth to a lot of crazy theories, predictions, situations, etc.
Assuming this Smash game doesn't get delayed and does, in fact, release this year, our speculation period will be noticeably shorter than even Smash 4's pre-DLC period.