Clarke was called up due to Watson's injury, but he's pretty much replaced Martyn now, with Symonds warming Watson's spot (and Watson being a benchwarmer for either MacDonald or Henriques).
In terms of replacing Hayden and Langer, is this the same Hayden that most Englishmen claim is nothing more than a flat track bully? It should be easy to replace such a character. Concerning Langer, he was hardly a key part of Australia's dominance, entered the Australian team in 2001 to replace Slater. Averaged 45 which was decent (cobsidering Slater and Taylor both averaged 42 at times of better bowlers and lesser bats). Their replacements, Rogers and Jaques, have both had long apprenticeships. Jaques is a highly aggressive batsmen who is perfectly willing to play his shots and likes to intimidate the opposition team. At 27 years old and with a FC average of 56, he's an opener in the mould of Hayden and Slater. Rogers, at 29 and with a FC average of 50, is a gritty and determined batsmen. A batsmen who is both shortsighted and colourblind (which causes him to occasionally lose the red ball if he doesn't focus hard), he has overcome great odds and is a battler through and through... wait a sec, this is starting to sound familiar. If you are familiar at all with Australian cricket, you should know that the one thing we have in abundance is quality batsmen. If you aren't, well now you know

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Clark is a great find. He won't be able to replace McGrath, but we don't mind. We're more concerned in replacing Warne. He will more than do the job required.
And yes, Macgill isn't Warne... he turns the ball further and has a much better average in India. He also takes more wickets than Warne when they are both playing at the same time. Doesn't have the control of Warne though, however, MacGill will do the job in the short term, all the Aussies are confident of that. What worries us is the development of the young spinners after MacGill goes. Trust me, when 2009 comes, the English batsmen will be having the same old troubles with MacGill, and unlike Warne, he can still bowl all his repertoire.
Gilchrist has been a one off, but we're not looking for a clone. Plenty of keepers in Australia that can keep and score runs.
A lot of people are retiring, Langer, Hayden, Gilchrist, McGrath, Martyn and Warne. Of those 6, 3 are important (Gilchrist, Warne and McGrath) and of those 3 Australia is only really concerned about replacing Warne. You're being melodramatic about Australia's position.