I'm unconvinced. The reason that, in the real world, guns supplanted a skilled warrior elite is simply that gunmen are easier to produce than knights. A knight (or bender) would have to train their entire life to be effective, which seriously limits the size of your fighting force. On the other hand, guns allow you to go grab the local peasantry, drill them for a few weeks, just enough to stand in a line and fire on command, and field a much larger force. So, even though a skilled bender would beat a singular conscript, an army using guns and benders would be much larger than an army using only benders.
Also, at multiple points in the series (namely the chi-blockers), non benders were able to fight benders, not necessarily on even footing, but still effectively. Putting firearms in the hands of such skilled non benders would only help even the odds.