It would be a fantastically interesting test of 'Aussies' and football and I would love to know the answer myself. I cannot see it happening while the aleague stays around though.
If the Roar were relegated I would be sweating on our games waiting for a chance to get back to the top flight - and I suspect I would lose a lot of interest in the stinking plastic franchise system and their fake plastic clubs until we got back up
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The owners would surely bail since there would be no aleagues licence to own in the current sense and with the drop in earnings from aleague mainstream media funding to likely bugger all second tier funding the departure of good players would be a greater impact than in a normal 'EPL to second tier' sort of drop and loss. From a quick Google search the whole EFL 3 tiers have a SkySports media deal for GBP 935 million over 5 years - which would be about 2.6 million for each of the 72 clubs per year if spread evenly. I can't see our media funding being that effective in an Australian second tier (again - we can dream).
I don't believe relegation should feature in a 13 team comp though. You need enough midtable teams to allow all 'decent' teams to stay up year after year as long as the club was willing to pay to have that decent team. A competition of only 13 teams that saw one drop each year would be a very hard pitch to sell licences and encourage 'investors' wanting to lose money on a football team. Someone has to be last and 13 good teams is not outside a realistic possibility. Imagine playing the best football in a season and being relegated from 13th instead of 18th. We need relegation to make us stronger rather than to just have it as a negative trophy.
On current form the Roar would be deserving of relegation. But in the current format the owners can afford to spend less and carry a couple of seasons that way to improve their financial stocks if they want. Bring in relegation and have 18-20 teams involved and the top 13 owners would no doubt be more interested in having decent coaches and teams every season.
Short answer though - I would stick with the Roar, catch the train to Perry Park (we are speculating after all) and like any second tier club would be looking forward to a time when we got back up. I expect a lot of big game/event fans rather than actual club fans would stick with the big league whichever team was in it. With fairly low crowds at our own games compared to what I want to see (10k+) maybe we are already down to the rusted on fans and there might not be that huge a drop.
On the flipside, teams coming up from our second tier would not be bringing the same number of fans with them so we would expect to see a drop in numbers across the league every time an established aleagues club was relegated.