Its not impossible, that's true. The writers can contrive whatever they like if they wanted. However uncovering these barely (If at all) understood techniques and creating something that does something NONE of those things do aren't the same thing. It strains credulity.
There are limitations in how magic works in Thedas, true, therefore no new things can appear out of nowhere without any explanation, but considering how many things we've already seen created or what feats magic or enchanting can obtain, I don't think the credulity is endangered. We're also about to visit the Tevinter Imperium - the most liberal place, when it comes to varied uses of magic. Heck - they even have flying cows there 
...And we have blades created by willing spirit bound to enchanted hilt. We have lyrium guns as well as means to travel distances through magical mirrors. Personally I'm not really that worried about credulity in that regard, with so many magical technologies being established and further developed. In any case - I suppose we're going to wait and see what happens...
My stance has always been that if Bioware wanted to continue the Inquisitor as the protagonist, they wouldn't have taken the arm. The arm, taken together with the quotes "My own adventuring days may be done" and "Then we'll find people he doesn't know" paint a pretty clear picture. It all makes it easier to excuse the Inquisitor no longer being on the front line.
Actually, before Trespasser got released I was worried about the exact opposite thing. With super-powerful Anchor - a unique power capable of passing to the Fade and even suggested by Morrigan to be capable of opening the gates to Black City - Inquisitor became too much of a demigod to let them just 'go about their business'. I was fully prepared for the DLC to end with either Inquisitor dying or travelling to parts unknown, which would effectively remove them from Thedas and most of the future story.
With Anchor gone however we're back to square one... well, only not exactly square one, isn't it? More like 1.5. Inquisitor leads a still powerful organization, has friends in highest places as well as access to both resources as well as arcane mysteries they discovered along the way, giving them rather clear advantage over almost everyone else.
Plus, even without one arm they should still be formidable - especially mages. During the course of Inquisition they've battled hundreds of enemies, killed a dozen dragons, fought the horde of darkspawn and a guardian of the Titan and at least two pretenders to the title of a god. Taking that into account, crippling Inquisitors seems like a no-brainer, especially if they want them to return as a protagonist (even as a limited, secondary PC, which I think is more plausible) - that itself would be a valid reason to explain why they have to re-learn how to be formidable again.
And actually - the "my adventuring days may be done" exists only in one option and appears to be said only to placate the public. Highly approved Sera's cards especially have sections suggesting that Inquisitor being "desk bound" or "leading boring life" is a ruse.
Also - an angry response (one that disbands, I think) actually ends with "now excuse me, I have a world to save".
Then, there's of course, the post-epilogue scene showing that Inquisitor will be very much active in some capacity. Heck, even their LI can appear in that room. I got my mage Trevelyan with Cullen and while their epilogue cards suggested that they did nothing in particular, Cullen returns and stands behind Trevelyan wearing Inquisition's red outfit.
And yes, I know the Inquisitor can say "I'm going to save the world" instead of saying their adventuring days are over, but the two don't contradict. Saving the world doesn't have to be done from the front lines.
True. But I don't suppose that conflict or attempt to stop Solas will resolve far away from front lines.The implication is that Inquisitor has made it his/her personal mission to stop Solas - be it at any cost or convince him to change his mind, as a foe or a friend. Basically, at the end of Trespasser, instead of their arc ending, they were given a new one, which is actually more powerful and compelling than that of stopping Corypheus.