I think main innovation in ROS2 is not necessary DDS but RMW (see ROS 2 middleware interface design document).
It allows to decouple ROS2 from DDS/Zenoh or any other protocol. Which brings me to the idea that ROS2 can implement new RMW which would be based on TCPROS (which is ROS1 is based on) so people who prefer ROS1 for out-of-the box performance can use it.
There is RMW based on SMTP already, so why not have RMW based on TCPROS?
Another idea is that default ROS2 DDS profile is not suitable for loads with small messages when latency is important.