My humble opinion...

Australia may already have some very good replacements. McGill for Warne (others I probably don't know)... Johnson, Tait and Watson for McGrath...(and once again, others I may not even yet know of).

But in cricket, as in other international sports, there is a limited number of names that are accompanied by a certain "aura".

Think of what happens in a discussion when you just mention names like: Brian Lara... Sachin Tendulkar... Inzamam Ul-Haq...Muttiah Muralitharan...Shane Warne...Glenn McGrath...

Suddenly the discussion becomes more animated. Comparisons are made. Statistics are being flung around like waywardly thrown frisbees. Why? Because of that je ne sais qoui. (for lack of a worthier phrase)

Their team-mates depend heavily on them, the opposition fear, loathe and respect them all at once.

Yes. Replacements there will be. But will they have this same "aura"? Will they have the same effect on the opposition before a ball is even bowled?

No. Not until they prove everyone otherwise.