I don't see how it is argued that the game cannot sustain another professional club due to an uncertainty regarding where the players will come from, or the perceived lack of players of a sufficient standard.
I think Australia has 50,000 adult players. UK estimate 25,000 players? Add in PNG, NZ, France and the other smaller nations and there's probably a pool of at least 100,000 adult players from which the professional game will draw the best players.
It's only ever a tiny proportion of these players at the top of the pyramid who become professional.
If we add another professional team, then it's simple; another 30 players are extracted from this pool and earn a living from the game.
For each new team, the quality at the top may be diluted, but it's such a small number of additional players as a proportion of the worldwide participation, that the dilution in quality would not be felt.
What would actually happen when a new professional team is added:
1) Elsewhere across all pro leagues a few extra junior players will be signed on
2) A few players may earn a contract for longer than they would have previously (instead of being 'released' or 'retired' the players would play for a little longer)
3) A few additional amatuer players may transfer into the professional game
4) Possibly a few additional cross-code transfers into the league code too to match the increased demand and opportunity.
The effect would certainly not be felt on just one league (example league 1 where the new teams may enter). The effect would be diluted across all leagues across the world and thus become negligible in any individual league.