I agree with this although I don't agree that Rio would be blocked by the ACCC. Why would they care when there are no Australian customers and the global market is so much bigger than these two? If the Colesworth duopoly is OK then this should romp it in.
The ACCC takes a dim view of #1 and #3 merging in the market (the market being the Pilbara). #2 and #3 combining will be ok. They'll be outcry from customers regardless, but less so when the takeover target is close to insolvency.
I reckon BHPB will make a play not long after the South32 demerger.