If I remember correctly, the ECB said that major market participants had asked for a third option for connecting to T2S - linking directly into the ECB's core network.  Taking that major market participants probably account for 80% of volume, that would leave around 20% of all T2S messages being divided between all of the remaining market participants and the other two methods of accessing T2S.

One of the beauties of open industry standards like ISO 20022 is that messages developed from that standard can run over any network, including even the public Internet.  Open standards also make it much easier and less costly for firms to link directly into service providers rather than having to go through messaging hubs.  Going via a messaging hub tends to be more cost-effective for organisations that have very low volumes of messages.  The approach that the CSD consortium is taking in this case would appear to be a reflection of the volume of messages that they expect to route through to T2S.