That includes an interesting observation about using the SASR because of a perception at the political level that only its personnel had/have the ability to carry out any number of tasks with any competency - that is, without inordinate (or any) casualties.
At first sight, that calls into question the rest of the army. If it's not fit for purpose, then what is it there for?
It's a ridiculous premise, of course, but it was allowed to gain a hold.
For contrast, Andy McNab observed of the British Army guys in Iraq that they were doing method of entry stuff that would have been the exclusive preserve of SF a decade or two back. Small-unit tactics, arguably, were refined to a peak during the period extending into operations in Afghanistan.
Perhaps another question to ask is who promulgated that perception within/about the ADF?
Re what I have bolded in your post: I've been on the verge of a post on this aspect so I'll throw a few words down now and maybe add to them later. In summary I think it comes down to Corps/Regiment lobbying in Canberra in order to raise certain units' involvement and benefit mid-ranking officers' careers on the way to getting 'their men' into the top jobs. First some background to explain, hopefully, my perspective.
I left the British Army in 1983 as a SSgt and joined the Australian Army as a Cpl in 1983. I won't crap on about reasons, but when the CSM of the first unit I joined (RIP Acko) took one look at my (small) accumulation of medals and declared "you're going nowhere, get to the back of the queue" I was not unhappy. And going nowhere outside Australia was easy because no one in the '80s was going anywhere bar the odd captain to UNTSO/UNDOF.
The Australian Army was (and still is) very small. Officers at least mostly know each other. WOs and SNCOs know many of their peers in other Corps and Regiments. As I got posted around in the '80s I became aware of corps rivalries in the corridors of power. Also discussed, even amongst NCOs, was VSO careers, who got promoted and why. It was generally agreed that this was due to contenders' Corps of origin, Vietnam service, whether decorated in Vietnam, whether SASR in Vietnam and so on.
Then in 1989 we did start deploying. We had for years been on the proposed ORBAT for UNTAG, the UN force tasked to oversee the transition in SWA/Namibia. Our contribution was to be an RAE construction squadron. I went with the second contingent as the CE's Int analyst. It was probably at that time that I started to learn about how the contingents for deployments are created, specifically how cap-badge rivalry is involved. As I said, in Namibia we were a construction squadron which comprised two construction troops and a field engineer troop (also a workshops and national HQ well to the rear). Such was the lobbying by RAInf back in Australia that if there had been another rotation the FE troop would have been replaced by an infantry assault pioneer platoon. Such was the bitter determination of RAInf to get even the smallest element deployed no matter the loss to capability of the deployed force.
As we moved further into the 90s deployments increased: Cambodia (large RASigs force with no attached Int/CI), Somalia (Inf bn with every individual Int attachment scrutinised to see if it couldn't be replaced by a 'bayonet') and finally East Timor which saw everybody and their dog get a guernsey. And that's when things started to change again.
Pardon the chronology above, but anyone still with me will have noticed someone missing; it wasn't until East Timor that SASR got a gig. They made sure that they kept their foot in the door and then got a serious part in Iraq 2003 including, IIRC, successfully seizing al-Assad airbase, which sounds like a commando or air-lifted infantry task to me, but as I said, the foot was in the door and then just as Iraq calmed down (lull before the storm obvs) Afghanistan took off. It appears that SASR pressed the right buttons to get the gig when a large part of what they ended up doing could/should have been done by Cdo or 'ordinary' infantry. But by jealously guarding the deployment, SASR dominated the honours and awards lists and cemented their position as 'blue eyed boys'. There was grumbling 'around the traps' as we say about this from a very early stage, perhaps 2004 or '05.
In the new century the SO world expanded with the unlinking of 2 and 4 RAR and 4 RAR being designated 'Cdo'. IIRC 4 RAR Cdo then became 2 Cdo Regt with 1 Cdo Regt being the Army Reserve unit. More SO units meant more positions, not just for officers but that's what I'm concentrating on here, and greater prospects of promotion for those officers. This is what perhaps (perhaps) saw the beginning of what we have seen in recent years of ex-SO officers working their way up and filling the very top positions.
Our initial involvement in Iraq was very limited and we were kept away (as far as was possible) from any dangerous places/activities. It was after my time in the ARA, but the eventual deployment of a Battle Group (with, I believe, restrictions on what they could and could not do) was due to RAInf lobbying as was the eventual deployment of a Bn to Afghanistan to conduct ANA mentoring.
I understand limitations being placed on forces for political reasons, but I could never be comfortable with force composition being arrived at by Corps/regiment lobbying.
All of the above is intended to shed some light on how I saw things in the Australian Army. I have tried to exclude bollocks as far as possible, but I might be guilty of rambling.