That's my question I have planned to answer in this post.
I believe that nothing good can last forever and believe the Aussies are now at the same stage the West Indies in the late eighties and the nineties. The Australians are not the invincible machines they once were and that is mainly due to the following reasons.
1) The Australians played with greats such as Mc.Grath, Langer, Warne, Martyn, Gillespie, Gilchrist, Fleming, Bevan, Steve and Mark Waugh. But now as these greats have retired the Australians have struggled to replace them. The machine like precision of Glen Mc.Grath on the off-stump in the corridor of uncertainity is irreplaceable so is Warne's mastery of leg-spin. Jason Gillespie, Damien Fleming and Glen Mc.Grath always proved more than a hand-full and even the world's greatest batsmen have struggled against them. Though Brett Lee has taken to the task of spearheading the attack well, it remains to be seen his success in the sub-continent. The Australians are struggling in the spin department. There's no one to fill the big shoes left void by Warne. Even the batting is not the great it once it was. Though Ponting has led the Aussies to two world cups and is a natural leader like Steve Waugh, and Micheal Hussey has succeed Bevan, the Aussies still struggle. Langer's place as opener is still open though many have been tried as openers. The weakness of the batsmen from the down-under against quality spin bowling is clearly visible as none use their feet as Martyn did. The role of a keeper-batsmen is also largely at want. Haddin is good behind the stumps like the keepers befor him as Adam Gilchrist and Ian Healy, but his time in front of the the stumps has not been able to lift any spirits. There are still many more places to fill and it is seen crystal clear that the Australian bench strength is not really strong or even close to it.
2) The second reason is because the gap between the 'Invincibles' and the rest of the world is shortening. India, Sri-Lanka, England, and the South Africans have well balanced teams which are more than able to compete at the top level constantly. I feel only Pakistan have deteriorated. Even New Zealand and Bangladesh are doing better and becoming more competitive.
3) The third reason I feel is the incapability of the Australians to adapt to different pitches and conditions. They have done well at home but have been beaten by India at WACA Perth, the very place they consider their fortress. The Australian batsmen have been guilty of not being able to adopt to the conditions abroad. They found it hard in the Caribbeans and also now in India. The slow pitches of the sub-continent, the swinging conditions of England and also the seam and bounce of the South African pitches have been places of struggle. The batsmen have also been guilty of not being able to adopt to the situation. The present team is lacking in experience and when a situation arises hell breaks loose. They also have been not able to implant the plans discussed in the dressing rooms successfully.
4) The forth reason of decline is because of the enormous pressure on the team. The batsmen and the bowlers are expected to give their 100 per cent and are scrutinized whether they are. The team is also expected to win almost every time they set onto the field. The decline in the bowling quality has increased demands on the batsmen. The team now requires the batsmen to score those extra 50-120 odd runs since the bowling depth is in adequate. This causes the batsmen to come under a lot of pressure. The Australian also have not been able to maintain their high run-rate they once used to. The hectic international schedule also takes its tole on the players.
5) The final reason I believe is the quote that 'No good thing lasts forever'. Even the mighty empire Rome fell and it's not far when the Australians too do fall. This is exactly what happened to the invincible West Indies of the 70's and the 80's. I like to conclude my reasoning quoting Steve Waugh,
The thing we learn from history is that we do not learn from history.