This kind of business has very strong network effects and both...

This kind of business has very strong network effects and both REA and DHG are established players owning between them a huge share of the overall market, so I think barrier to entry is quite high at this stage. Note network effects work both ways - REA/DHG is where people look for properties, so that to large extent forces the agents to advertise there in order to reach wide audience. A group of agents trying to set up their own business just wouldn't cut it at this stage, without some serious disruptive idea/advantage.

My guess is if Cat was looking for a challenge building an online real estate business, he'd stay with DHG - they have plenty of challenges ahead in order to grab market share from REA, which is the incumbent after all. So unlikely scenario IMHO.