The reason we have 'Holden' brand is that after WW2 we had a lot of manufacturing infrastructure that could be used - folks like Chifley and Harnett had a vision and voila - Holden. Seeing 50% market share for medium-large 6cyl cars, Ford and Chrysler got in on the act.
Our 'car culture' based on muscle cars, Bathurst 1000 and V8s stems largely from the period 1967-72; this was the classic time when Aussie car scene was at its peak. This closely ties in with what happened in USA. Then you had....supercar scare.....Energy crisis....arrival of decent Japanese imports, and a burgeoning local manufacturing scene focused on Japanese makes....the traditional hot-rod orientated car scene was under threat. In the 70s you had XA-XB GT, V8 Toranas.....yeh it was still going ok. Then you had XC Cobra, A9X Torana and arguably with the 1979-80 energy crisis MkII, Valiant folding and Ford basically giving up muscle car aspirations and set out to run a profitable business. Via Brock, GMH outsourced its special vehicle division. Now Brock and Holden had a great realtionship around this time but from '85 onwards Brock stopped winning enmasse. In '87 the business model went bad as Brock went down the energy polariser path.
By now you had Group A touring Cars, and the V8 Commodore was more or less outclassed until everything that was better than a V8 was banned after '92. I would argue that any V8 Holden vehicle developed from the late 80s onwards was focused on a niche, enthusiast set that has strong aspirations of nostalgia.
For so long GMH had been a one trick pony; one model line for its first 19 years, 6 with two then 5 with three. During the 80s GMH didn't know what they were doing. Camira, Piazza, Scurry, Drover, Astra, Barina - no wonder Holden nearly went broke! Just as well here was a nice veil of economic tariffs to protect Holden as a company from other car companies who knew what they were doing. Oh no, wait - the Button Plan from '83 sought to reduce tariffs, and rationalise the car industry. No other country of 15 million people (at the time) had two (or 3 counting the Magna) models lines developed especially for them, with very limited export potential.
Fast forward to the 90s, Ford and Holden plants are being wound up one by one as market share drops. Toyota is doing well. Nissan closes up in '94. By '95 all you have left being assembled is Commodore, Falcon, Camry, Magna, and the Corolla. Fuel is cheap, the new V8 Supercar series gives Ford and Holden new marketing potential but....its not the same. There are muscle cars (HSV and Tickford) but they are now priced as high-end luxury cars. Not really Muscle. In '99 Corolla winds up, as a new platform (vastly improved mind you) is launched from Toyota City in Japan (what have satellite plants when you can consolidate?).
Meanwhile going into the 2000s, small cars are getting better and better in terms of space and performance. Tariffs are falling, so they are getting cheaper. The SUV segment becomes a force to be reckoned with. Now Ford gets in on the act with the Territory, but it can't really compete with SUV expert brands like Nissan and Toyota. In from 2002-2008 fuel prices accelerate out of control, killing off demand for big cars. Mitsubishi dies in 2008 due to poor sales. The VE launched in 2006 is the right car, at precisely the wrong time. Changes to V8 supercar rules effectively make it a mockery as cars deviate wildly from basic structure of a regular Falcon/Commodore. Then the GFC hits - people buy cheaper cars (smaller cars) or don't buy new cars at all. Whilst this is going on, fleets are fleeing large cars in favour of...yes, small SUVs and small cars.
While this is going on, internet, multiculturalism, feminism and general globalisation is unseating the white aussie male as the centre of the Australian economy. Women are having an equal (or more?) say in buying cars, which means either a small car or small SUV is more likely to be on the menu. People identify more with an international and/or cosmopolitan lifestyle of sophistication and luxury - this is not conducive to getting people to buy a V8 Commodore ute. Holden uses some of its spare assembly capacity to build the Cruze bit it can't compete with the Corolla in a crowded marketplace. In the end, Holden decides that it would be more profitable to just import cars rather than build them here.
Basically.....its not 1967 any more, and the world and society has changed. If enough people wanted big 6/8 cyl cars, I am sure Holden and Ford would build them. But this is not the case. Humanity in general has turned its back on the 'bogan', with GMH doing what it can to stem the red ink. Its business model was ultimately reliant on a shrinking market segment.
Now local manufacturing has been a core part of Holden's identity. Without it....Holden is now an empty shell. It may well be the case that GM is pulling out of all RHD markets. They wouldn't sell anything in Japan, not much in India and UK brand of Vauxhall is now gone to PSA.
That being said - I wouldn't be surprised if Holden is replaced by Opel (or Vauxhall) - it just won't be owned by GM.
OTOH - Ford seems to be going fine, globally. They have a good range of cars that can be suited to RHD markets. Massively popular in the UK.