I'm not an expert, I've just read a bit. Others know more.
ADS-B systems use the 1090 MHz Extended Squitter (1090ES) data link.
ES not not universally used. The prime advocate of it is Australia. This is one of the reasons we have trouble getting ADS-B capable transponders.
Also your diagram shows ADS-B in, which Australia is not implementing.
The document that your diagrams come from is 14 years old and is predicated on data transfer speeds of 1 Mb/s. I'd be very surprised if there has not been significant improvement in these speeds which will increase the data capacity.
Congestion depends on the amount of traffic. Australia is the only country in the world that is universally mandating ADS-B for GA IFR aircraft. Other countries implementation varies. Some are by flight type (ie RPT) and some are above an altitude.
Only Australia will require all IFR aircraft to use ADS-B.
ADS-B is another example of Australia thinking it has to be different than the rest of the world and the cost of this is borne by aircraft owners.