Darwin also doesn't have Perths 120 000 UK expats living there, nor a population of 2 million plus to help provide an economic foundation for the services, which I believe will primarily focus on traffic out of Perth, Adelaide, and Southwest and Regional WA. Traffic from elsewhere is obviously important, but with more choice and more convenient/cheaper connections through Asia or the Middle East, you wouldn't want to rely on it.
As far as alternates, there are plenty of 45m wide, 2km or longer runways that are reasonably close with adequate facilities and parking; Kalgoorlie, Geraldton (if not already wouldn't need much of an upgrade), Pearce (if its not Weather related), possibly Busselton in the near future depending what they do with the soon to be commenced widening and lengthening - then of course Adelaide, Learmonth, Port Hedland (and maybe even Karratha - parking dependant) if it comes to that. The Cat 3 system should be operational soon enough anyway, which will vastly reduce and minimise (not eliminate as some believe) the days where many alternates might need to be carried.
If Perth Airport wants these services to operate from the International side, and if the State Government says that regardless they should move there within 5 years or so, then stump up the cash and get on with constructing Qantas a domestic terminal over there. Bring forward the terminal consolidation plans. Otherwise, the $25 (odd) million gives you 10 years to take your time, and it gives the Qantas Group its own terminal space, both International and Domestic, to do what it needs to do do grow in the mean time.
Virgins new terminal took around 3 years, and cost $212 million dollars. So if they got cracking on it now, factoring in the usual delays and faffing around with various silly business lobby/govt/environment groups etc, that we deal with in this country, you could have it done in around the same timeframe. I'd reckon Qantas wouldn't be ready to commence these services much sooner than that anyway, given the delivery schedule of the aircraft, and that they are probably going to need more than 8 to do everything thats currently rumoured.