Firstly it's a weak excuse to say it's only happened in rubbish name space. What about .ca, .jp ... I can go on it's a long list, .nl, .at etc etc. The facts don't back up the statements.
Next people tend to only focus on a model where allocation based on rights and or payment exists. You may have a model that dones focus on that and looks like:
1) All domains with .com.au enter reserved list in .au Registry for say 5 years
2) Within the 5 years if the .com.au expires/drops, then the .au becomes available for normal registration. First in best dressed.
3) If within the 5 years both parties who hold the .com.au/net.au domains consent to allowing the registration of the .au, one party gets first option. You'd find they would simply have a private discussion, some agreements would take $, some wouldn't.
4) At the end of 5 years, all .au domains open to normal registration - first in best dressed just like say the drop service runs today. I'd imagine all registrars would participate in the process, not just Drop/Netfleet. A fixed time frame stops an ongoing stalemate between the .com.au and .net.au owners.
I think the other things to consider are:
1) The new tld's if they are wildly successful (and I'm not optimistic but you never know), may well pressure auDA to look at opening .au regardless.
2) Opening up a new space possibly diminishes value in some portfolios, but opens opportunity in others. I don't think domainers who are generally smart with regards to long term domain value will lose out, in fact they'll probably do well.
3) Most arguments are based on retaining existing domain policy or .com.au and .au would be aligned. Maybe com.au stays as is, maybe .au is completely open to anyone. Or .net.au has all policy removed so it's differentiated and drives growth in the space.
Do I see challenges? Absolutely. Is my model above brilliant? Not really, it's just one way it might look, there's 10's and 10's of options available, I think most big hurdles can be solved to the majority of domain registrants.
In a lot of respects though having been involved in this debate many times I think it has a lot of parallels to the .xxx debate.
You had the porn industry aligning with the ultra conservatives both with their own different agendas (porn didn't want the value of their .com real estate diminished, the conservatives just didn't want porn on the internet period). So a strange marriage. Both parties emotionally argued if .xxx was allowed the sky would fall in. Clearly .xxx happened and the world went on as usual for all of us.
Here we have domainers aligning with the ultra anti-business types who feel domains are a utility, should be free and the internet not commercialized at all (obviously the opposite of the domainer community), ultimately taking the same position just like .xxx. I feel just like .xxx the arguments of the sky will fall in will eventuate to zero if it ever happens. Life will just go on.