I don't know. Maybe it's just because I played a two handed weapon user, but the loss of an arm being the thing that sidelines my character makes sense. He'll just never be able to fight at his fullest potential again. He'd still probably be able to handle himself with a longsword, but there's a difference between handling yourself and being able to save the world.
And I like that. It's humanizing to a character that needed some humanization; to have suffered this wound and have to suffer the realization that he has to step back away from the front lines. Not to mention that having been crippled by Solas does a lot more to fuel a personal grudge than having been mildly inconvenienced until Dagna could work up a replacement.
You're really stuck on that two handed specialization 
I wish we could have a DA4 where you solve your problems through intelligence, manipulation, strategy, persuasion, teamwork, preparation, stealth, and combat rather than combat alone. Being a one woman murder-machine as your only means to solve problems is boring anyway. I hate RPGs' over reliance on combat.
I very much disagree that losing an arm would mean you can't fight anymore though. You may not fight as well, but if you take this super strong demi-god of a character that plows through bandits, elite fighters, dragons (and even bears!) like tissue paper and make that person much weaker, then they'd just be an exceptionally skilled normal person. Combat would be a challenge rather than your win being a given.
As far as a prosthetic (or what I want which is a simple Captain Hook style attachment of a dagger or something) making losing an arm a temporary inconvenience rather than a real physical and emotional challenge...try saying that to someone in real life with a prosthetic limb.