Several points here:
- It is by definition the captain's job to take the team he has on paper and make them perform as a unit and get as good or better results than you would expect out of them. If he fails to do so and they underperform as a unit, then that is a legitimate criticism. Yes, there are other factors, coach, staff, etc, but in the end, the best assessment of a captain's merit in my book is how effectively he improves the performance of the team under his helm.
- The 2005 team were favorites to win, yet ended up losing the series. On paper, even without McGrath, they were arguably the stronger team. So it should never have been that close a series to begin with. England were superbly led by Vaughn but they definitely punched above their weight whereas Australia were on the backfoot after the first test. You can argue that one or two players lost form, but the point is that under a truly great captain, they manage to get the team collectively to lift their performance when faced against similar opposition.
- This doesnt mean that every loss means a demerit for a captain. Obviously, NZ under Fleming and Sri Lanka under Ranatunga could not be expected to beat Australia in their backyard, but if the team's performance can be made more competitive under one captain then credit to him.