The fact that you're asking for an explanation of how diminishing returns will effect the NRL's market in Brisbane means that you don't understand it.
Look it's not hard. Putting it as simply as possible (probably a little overly simplistic but whatever, it's suitable for our needs) the law of diminishing returns is a principle of production that argues that changes to a single factor in production will have a reduced return on investment each time you increase it so long as all other variables remain the same.
Using the NRL in Brisbane as the example; increasing the amount of teams increases production, but the cost to add each new team will remain stagnate while the increase in production per team will diminish with each new team until you hit a peak after which it'll actually decrease the return.
In other words the addition of new teams will cost the NRL the same or more than adding the Dolphins did, but the return they receive for that investment will be smaller each time they do it. For example the Broncos cost e.g. $15mil a year and average 30k a season, the Dolphins cost the same and average 25k a season, Easts cost the same and average 20k a year, and so on until you hit a peak after which point it starts to impact the whole line of production and you decrease return across the board (i.e. everybody's attendances start to go down).
Marry that with other basic economic and market factors, e.g. supply and demand, market saturation, etc, etc, and adding more teams to SEQ in quick succession is just quick marching the Brisbane market into being in a similar space as Sydney's, and that isn't in anybody's interest.
And yeah it's inevitable that the law of diminishing returns will effect the the NRL on the national scale at some point as well, but the NRL is a fair way off that point just yet. It's starting to impact the AFL though if you want an example of it in action in this industry in another sport.
Does that clear things up? Probably not right, but a man can dream lol.