Just so everyone knows, I don't think it was confirmed that the Inquisitor wouldn't return as a protagonist in the sense that Bioware directly stated those words. It was confirmed by their stated Trespasser goals including "concluding their story" and "tying them off". Which as Nefla said, they need to brush up on their definitions for. And here we go again! Yay?
So how do you expect the Inquisitor to wield a sword & shield with only 1 arm? Or dual wield 2 weapons? Or fire a bow? Or cast spells?
Fairly easily, all things considered. First, there have been people who continued to battle in our world with prosthetics. Hundreds of years ago. Any prosthetic in Thedas can be much better. Second, you don't even need a prosthetic, as Nefla's cute drawings show. Here's one to finish the set:

It would be just as easy for a shield or dagger. Remember the Red Templars? Their Shadows have literal arm-blades. A 2-hander would be a bit more difficult, but workable. And as Hazegurl said, it wouldn't affect a mage in the slightest, combat-wise. Mages don't actually need staves, and they don't even need to use hand motions to cast spells. They're like training wheels, which is why the most powerful mages we've seen don't use them (Flemeth, Solas, The Architect, and Corypheus don't use staves, Solas doesn't even use hand motions). Alternative options like spirit arms are also feasible for a mage, as Illyria mentioned.
A staff is like an enchanted sword. In fact, an enchanted sword can serve the same purpose as a staff (Arcane Warrior sword in DA:O). So the Inquisitor's prosthetic could even serve that purpose; put a charged crystal on it. But they don't actually need one. And think of hand movements as a semi-conscious action, like body language. It helps the mage convey their thoughts, but their thought is what matters. Like the Force, not like muttering an incantation in Harry Potter.
The lack of forearm really isn't a reason not to have them return at all. As Nefla said, I think it would be a great and unique opportunity to experience that sort of trauma as a protagonist and react to it according to our RP. And it definitely doesn't mean they can't do a dual-protagonist story.
Another thing to remember is liking the Inquisitor is completely irrelevant to wanting them to come back, contrary to what many people think. I found them fairly bland until Trespasser. I want them back to finish the story they started in Trespasser. Their story was finished in the base game, but apparently Bioware's definition of "tying off a character" is to have them go from staring at the sunrise to vowing to take further action against someone they're personally invested in, and actively plotting against them.
I've written loads more, and better, but it's hard to find a post from months ago.
Businesses want to retain their current customers and acquire new ones. Why would they willingly drop paying customers to seek out potential new ones?
You sure about that? 
That reminds me of Velanna, whose quest I could never get to trigger. There's still no real consensus on what causes the problem and looking for answers is a nightmare -- until I found yet another helpful thread here. I found a solution that is incredibly wonky and ruins the narrative, but it works. It's in no way an "official" fix and can't be found on wiki.
BioWare, don't be prideful. Just reverse this closing.
Edit: my typos were atrocious. DX
They have a huge problem with that. Massive. *sigh* 
On the whole closing issue, it really feels like we're an "inconvenient part" of their audience. And even if we're a small part, the whole thing seems really extreme. Not gonna repeat what tons of people said about how forums allow discussions that SM can't really replicate.
That may be the saddest part of this situation. That the most passionate, invested fans are the most inconvenient to them. I don't even.
I didn't even know who he was until the closure of the forums was announced.
I didn't know until they closed off-topic. So your point still stands.
You know, to make sure BioWare games are playable for everyone. Not just the relative minority of posters here. I love encouraging people to try to play games I think are good. It's a lot easier with one focused site. Am I yelling into the wind? Probably. Is that going to stop me from trying? Heck no.
That's the spirit! Push on to the inevitable. For our principles, if not for victory.
Keep up the great work, people.
And if we get deleted, we'll get deleted knowing that we did everything we could to stop this. That'll have to be enough.
*snip*
I don't think those new players will buy merchandising or dlcs. At least that is what I think, maybe the mouth to mouth was good for the old Bioware but not for EA.
Anyway, I think I'm done with Bioware and EA, I'll still play their games but I'll wait for a good discount. 20-30€ for a relatively bug free game seems like a nice price.
This forum will need some mouth-to-mouth in 2 weeks.
*shows herself out*