How is it a straw man? The exact same argument -- "Rondo was hurt" -- has been made dozens of times on this blog.
There are better counter-arguments than "Rondo broke his arm, thus we wouldn't have won anyway", but you chose this easy-to-dismiss one, which only superficially represents the position of people who support The Trade<tm>. As I happen to be one of those people, I should know. I consider rigging an easy target for yourself to be setting up a straw man, and the definition of the fallacy agrees with me. So there you have it.
However, there was no inevitability to that injury, as you admit. It's very possible -- even probable -- that if we still had Perk, Rondo never would have been injured.
As I said: if we grant you your alternate reality where Perkins was not traded, it's true that the extremely specific injury to Rondo very probably would not have happened. However, all bets are off in terms of other freak injuries to Rondo and/or other players, and resetting the board makes such things possible.
I mean, think about it: the freak injury to Marquis plays into this as well. Danny would have had less incentive to go big with OKC, if he didn't also need a backup wing. Seeing as how Marquis paralyzed himself on Gilbert Arenas' chest, certainly the Magic/Wizards trade played into that, which in turn was partially a reaction to Dwight Howard's performance in 2009-2010 against... Kendrick Perkins. Now, if I continue this line of thought, I'll eventually wind up in "Ronald McDonald uses cheese soup to control America from a secret Volkwagon in the past" territory, but hopefully at this point I've adequately demonstrated the madness of chasing down the Butterfly Effect.
I guess where I am going with this, is that I think we ought to let sleeping dogs lie on the Rondo Arm thing.
There's no guarantee that we would have done better with Perk, but I think the likelihood is high that we would have, especially in light of the fact that Danny apparently knew that there was a strong chance that Shaq wouldn't be back for the playoffs. Danny traded the starting center from a lineup that had never lost a playoff series. Doc has admitted that that hurt the team, and said that he wouldn't do the trade at the deadline again. I think it's fair, then, for people to think that Danny's decision hurt our chances.
Fair to think it, sure, it's fair to think anything that you want. This soldier, though, believes that to assert it, to state it as if it were a fact, is incorrect. There's enough meat on this bone for us to argue about The Trade<tm> until the heat death of the universe. As evidenced by this thread, and dozens of others like it.