But surely we'll get some sort of resolution to their character arcs even if they don't actually appear in game right? It would be incredibly disappointing to have a bunch of good endings for the warden (albeit an unsatisfying ending for a Hawke who sided with the mages) that get overwritten by a mysterious "they disappeared" with no actual explanation, only to never have it addressed again. Simply making the protagonist of the past two games disappear is an incredibly disappointing conclusion to their story arcs -- and yes, I think story arcs that players invest 40+ hours in should be reasonably well concluded.
On advantage of "they disappeared" is that it's open ended. If you'd prefer, I could suggest that we bring back the warden, and have the Warden become deranged with taint so that he/she actually ends up killing a small village and feasting on their bodies before being struck down by a bunch of dwarves.
Extreme example used to illustrate that if we have ultimate control of those characters (which we now do, given that both of those games are over), there WILL be outcomes that people won't like because in their minds "it's not what their warden will do."
My statement is more along the lines of expectation: it's a land mine to deal with in a lot of cases and if you go into DAI with some high expectations for what to see from Hawke and the Warden, there's a good chance you're setting yourself up for disappointment because DAI's focus will be on the Inquisitor's journey.
I can understand disappointment about Hawke's being rather abrupt and disappointing, and in retrospect we could've done something different as the plan had a larger expansion pack coming out, but reality nixed that. As such, Hawke has perhaps more loose ends and would probably have greater consideration for having his/her plot wrapped up on that fact. But at the same time the story is still about the Inquisitor and the events surrounding Inquisition.