Except that people would've been very angry to see their characters die at the end of their story. Removing the arm is effective at removing the Inquisitor from a combat role, gameplay-wise, but their story can continue, whether on-screen or in our imagination, doesn't really matter. It is *very* different from death.
I don't think we'll see an Inquisitor with a prosthetic as protagonist, simply because too many potential players wouldn't like it. Neither would I, actually, except if the new limb was an enhancement. The arm itself isn't that significant, but it would always remind me of what I lost in Trespasser, which was much more than the arm.
What I meant was if them removing the arm was some vain method of suggesting to the players that the Inquisitor could not return as a viable PC (I'm not saying they should return, but I'm not delusional enough to say that "Trespasser" didn't make the notion a compelling one) than Bioware failed in the worst way possible. Bioware failed to show the gravity of the situation, failed to show the drama or conflict over its loss, failed even to even make the loss of the arm apparent (****, I had to rewatch the damned cutscene twice to believe what I was seeing). No one mentions it, no one even looks at it and they follow it up with the Red Jenny Epilogue with the Inquisitor having a prosthetic to further confuse matters. 
If Bioware's intent was to show perhaps the chance of the Inquisitor's return as a PC who continues to fight despite their new handicap, that would make far more sense narratively than if they removed it to knock them out of the running. Much of "Trespassers" dialogue revolved around stopping Solas (a man who just admitted to you and only you than he plans to destroy your world) and deciding which the best method of stopping him would be. Do you fall back on the Inquisitions Diplomatic Core and Spy Network (the military is completely 100 percent useless if he is actually in Tevinter) to help you find him and risk being compromised from within? Or do you take a more direct approach and abandon many of those resources by disbanding? Do they want to stop Solas at any cost or save him from himself? "Trespasser" proves to be rather inconclusive if Bioware's intent with the removal of the arm was to keep the Inquisitor a backstage player (or non-existant) for the next game. They chose to enhance the relationship (whatever that may be) with Solas, add a ticking clock element where everything the Inquisitor has worked for could be laid to ruin if they don't continue their pursuit of Solas, and then chose to turn right around and cripple the Inquisitor assuming the players would see the handicap as a impassable wall for the Inquisitor to return in combat role, despite there being quite a few examples in the setting that would suggest otherwise?
If this was Bioware's goal they could have just easily done the following (with or without the arm removal): Solas monologues, removes the Anchor to save the Inquisitor's life and in order to keep them safe/out of his metaphorical hair he petrifies them until after his plan to bring the Veil crashing down is complete; because he's under the assumption that either 1) If he was a friend/LI with the Inquisitor that the petrification process has the greatest chance of preserving their life through the cataclysm he is about to unleash, or 2) He hates you so much if you had a negative relationship with him that he wants you to suffer so he preserves the Quizzy only so he can thaw them out at a later time and show them the ruins of the world they once held dear "you confirmed for me their was nothing in this world worth saving, I thought you'd like to see the result". The decision to disband the now Inquisitor-less Inquisition falls to Cullen, Josey, Leliana and Cassy (the four that started the Inquisition in the first place) and they can decide (through the player) whether it is better to shackle the Inquisition to the Chantry or Disband, either way hoping to find a method to de-petrify their leader and friend. There! This neatly indisposes the Quizzy, keeps the only person Solas valued or hated enough to tell his plan to under lock and key, gives a concrete goal to the Inquisition and/or its members, opens up room for a new Solo PC who could eventually be the instrument used to save the Inquisitor, and finally because the Inquisitor is the only one he told his plan to the new PC (and the rest of the Inquisition) would be made aware of Solas' plot only after they had managed to rescue the Inquisitor. Who, just after having the Anchor removed and turned to stone is in no condition fight Solas themselves, intrinsically having that task fall to the new PC. 
What I don't want to see is the Inquisitor become a simple cameo or quest-giving mentor. New players would have no investment in who this random ass-hole is (making them superfluous) and players who advocated for their return as a playable PC would just feel insulted. As it is now I would prefer a New PC not be one to handle Solas (as it would be far too similar to the Cory situation), but if that needs to happen so be it. Just leave the Inquisitor out of it completely if that's the case. Bioware, if your abandoning an interesting character dynamic for the sake of having a New PC ... go all the way with it. Plus, Bioware has shown to have a lot of issues bringing back previous PCs as NPCs. Hawke had three default personality types and two VERY similar endings and Bioware couldn't even get that right, how on earth do people think they could handle a far more complex PC like the Inquisitor/Ex-Inquisitor? 