Why are people surprised by this? They said ages ago that the plan was for every DA game to have a new protagonist.
Because the the idea behind that was that every game would be it's own self contained story in Thedas, which was true for Origins and 2. But which isn't the case anymore, as the writers said that the story for Inquisition was supposed to be twice as long, and we've only seen the first half. They have said that Dragon Age 4 is actually going to be the second half of the story that was meant to be in Inquisition. The actual plan for the plot of the next game was originally going to have the Inquisitor deal with it unless Inquisition was also originally planned to have duel protagonists which seems unlikely.
So, either way, one of their plans have changed. Doesn't seem impossible that they would forget the different protagonist rule for one game in order to tell the story they originally had planned. There's also the fact that everyone was fine with having a new protagonist for Dragon Age 4, as there was no reason for the Inquisitor to return, but then Trespasser went out of it's way to have the Inquisitor repeatedly declare that they were going to be involved.