Straight away you've undermined the club's identity as a Perth club.
Do the Swans play games in Melbourne, Lakers in Minneapolis, Rams in Cleveland or St. Louis etc, etc?! No they don't.
Why don't they do that; because keeping their finger in the old market's pie makes it look like they aren't committed to the new market and that they want to keep the old market as a backup should things go sour in the new one, and that's commercial cancer.
Besides, if the first game in Sydney was successful there'd soon be demand for more, and once the precedent had been set there'd be no rational argument against more games in Sydney so long as they're profitable.
At that point you'd have people, both internally and externally (such discussion in the media is also commercial cancer BTW), arguing that any game that they believe would or could be more successful in Sydney should be played in Sydney, at which point it's inevitable that more games would be moved to Sydney given time.
In pursuit of maximising profits the club would kill the goose that lays the golden eggs.