I agree that it's a good strategy, in fact the rectangular sports should have been doing it for years, but it won't work in Canberra's case.
Labour's indifference to the new stadium isn't about the amount of tenants, it's about the cost and their priorities for that money. In other words, the crux of the issue isn't that Labour think that another tenant is necessary, that's just another excuse in a long line of excuses. It's that they don't want to spend the money on the stadium until they have absolutely no choice, so they are delaying it for as long as possible to keep that money free for other things, the main example being the light rail.
Even with Parkes way being realigned the Civic pool plot will only just squeeze in a sub-25k seat stadium, which simply isn't big enough. So unless we're willing to knock some buildings down, there's not enough space in Civic anymore.
We (the rectangular sports) should give up on the Civic stadium pipedream and start pushing to get it built in one of the other city centres before they run out of space as well.
Woden is probably the best option as the next most central of the city centres, and it'd be a fitting home for the Raiders.