The decline of HEx simply indicate the success of EL - integration into the tube network, direct service to many more London stations, and reasonably comfortable ride.
HEx has potential to reborn by better integration into the TfL network. There are examples around the world that may be adopted, e.g. discounted or free connection to any Zone 1 station, through services to EL central London section, etc.
I think TfL should decide the future HEx soon. Ideally it should be tied with HS2, and the OOC station should be designed for seamless transfer between HS2 and HEx if they want passenger to go this way.