I will start this off by saying that I'm on the fence. I am VERY attached to my Inquisitor, more so than my Warden or Hawke, and would love to see him return in some form. However, I have previously been satisfied with the non-recurring protagonist concept as the model for DA games, so I won't be utterly heartbroken if they choose not to use the character. I'm also quite wary of how the whole issue of the arm would be portrayed, see below.
Hell, even though I adore Dorian to death, I'm not even sure if I want him in the next game. I do and I don't simultaneously.
It's interesting that people don't see the potential to invest more in the character because they lost a hand.
For players, I think there are several aspects of it, and each of those will apply to different players in different ways.
The first is that it can be a pretty heavy-handed (I'm sorry...) tool for the story teller, Bioware, to have the main character maimed in some way so it's easier for the audience to accept when they don't return. We saw the event happen in the game, which in some ways is much better than being told our Warden "disappeared" at the end of DA2, or earning that, regardless of whether they were viscount for a time, Hawke also eventually disappears.
I think that post-DAO Bioware is reluctant to come up with really detailed epilogues for fear of limiting themselves in future story lines. So some players see this as Bioware's signal that the Inquisitor is done, and some of those seem to have accepted that.
The second is the whole fact of losing a hand and what that means for combat and game mechanics, and resources in game development. This is NOT a story issue. Why do we have only a single color of a single model to use for the armless Inquisitor? Resource management. Granted, a DLC gets far less resource allocation than does the main game, so the limitation there is understandable.
But this would apply to the next game as well. Even if they know about having an armless Inquisitor in DA4 ahead of time, and have things planned out for that, things don't always work out like the devs want. Resources are limited. They would not just have unlimited money and time to make an armless Inquisitor absolutely perfect. How would they work out a missing arm? A prosthetic? Magic? I know I haven't been alone in not wanting an "easy fix." I bristle every time I see people throwing out the word "prosthetic" like it's some simple thing for a person to use, or even want to use. But even with a prosthetic the person would not, and should not, function as they did previously. It is not realistic (yeah, I know, "we're playing a game with magic and dragons"), and is frankly an absurd expectation.
So, how would this be implemented vis-a-vis combat and visuals? Going back to resource management, models with a single arm would have to be designed specifically for the Inquisitor, or an Inquisitor with a prosthetic built into the model. At best we would have a limited selection of armor models to choose, and at worst it would be like DAI Hawke and have a fixed body appearance, either with or without an arm. Then there is combat. I think mages would be relatively OK, but both warrior builds, and one-and-a-half rogue builds (I think dual-dagger can learn to compensate) would be more limited. Note that I don't say "impossible," but certainly not with the same function as previous.
Moving away from resources, forced use of a prosthetic takes away roleplay agency. And I would certainly be annoyed if we were forced to wear one without having the chance to offer any commentary on it at all. This arm deal is a huge thing for my Inquisitor, who is an SnS warrior. Daily life will be a struggle for quite a while and I don't want that ignored, and I do NOT want it left to head-canon. As I've said elsewhere, losing a limb is a big f-ing deal and should not be ignored by the game. It was bad enough that we didn't get to comment on it in Trespasser, but I lived with that because it was the grand finale.
The third issue moves away from the arm and just focuses on the Inquisitor's return in general. One word: Solas. For me, a strong argument for the Inquisitor's return in some capacity, even as a quest NPC or faction leader, is because they have more of a personal connection to Solas; friend, enemy, romance, it doesn't matter. The Inquisitor was manipulated by Solas and knows more about him and his plans than anyone else, simply from being around and talking to the man. I do worry that that missing connection will enable Solas to become some sort of caricature baddie and ultimately do harm to the story.