Yep, assuming that they were reasonably well run, an AFL side in Canberra would be as big as the Raiders if not bigger from day one. Their average attendance would quickly outpace the Raiders as well if an oval was built with a big enough capacity to sustain it.
The reason for that is, shock horror, there's more to Aussie Rules (AR) than just the AFL!
For historical reasons Canberra has a long history with AR and there's a huge bed of intergenerational support for the sport here. A huge portion of Canberra's population are expats as well, and a huge portion of those expats come from west of the Barassi Line (RIP). In my experience there's also significantly more overlap between the AR and RL fanbases in Canberra compared to other markets like e.g. Melbourne and Sydney.
Aussie Rules has more clubs, teams, and significantly higher participation numbers in Canberra itself than RL as well. Multiple of those clubs are highly successful businesses in their own right, particularly Eastlake and Ainslie. Aussie Rules is also very healthy in the private school system, which is a big deal in Canberra.
Just spending a bit of time traveling around Canberra and interacting with people here would make it clear to anybody with a functioning set of eyes and ears that Aussie Rules is very popular here, similarly popular to RL in fact, and it frankly just speaks to peoples ignorance when they suggest otherwise.