A Perth team carving out 3rd/4th place among the Eagles, Dockers and Force is fine. Also, no the NRL is not the largest or most prestigious competition in club rugby. The Heineken Champions Cup is, with it's average salary being nearly double that of the NRL, as well as 24 teams and broadcast world wide.
In fact I'd go further on the prestige, the RFU Premiership, LNR Top 14, URC and NZ Super Rugby would all be considered far more prestigious, and in the case of the first two fare more financially powerful, in their respective markets.
Die hards need to stop pretending that the NRL carries any major presence outside Australia and the North of England. It's a great game, but it's also a small game and there's no shame in that. The shame is in the small minds that operate it.
But this is a stadium chat, I would love it if Perth getting a team meant upgrades to HBF Park so that both Force and NRL Perth get a top tier rectangular stadium.
Also it's clear you haven't lived in Perth or New Zealand, the Eagles might be massive, but they do not attract the cult like following that the All Blacks do. Half of Perth despise the Eagles, ergo Dockers and other club fans. Plus the Rugby, Soccer and Basketball crowds who tend to hate them as well.