"If, instead, we had identified sessions using only layer 4 data, then mobile IP would have worked perfectly."

Mobile IP can still work with the current infrastructure -- https://en.wikipedia.org/wiki/Mobile_IP

This proposal was basically a service which would host a static IP for you (similar to the LTE structure but with IP underneath instead of L2), and forward to whatever your "real" IP was using IP-in-IP encapsulation.

As the author states, layers are only ever added :)