As we I know that the earth wire is not only for lightening protection, but it serve the fastest return path for ground/earth fault protection. Though each Tower structure is grounded, but the earth resistivity between the fault point and source point is very high. In fault condition Earth wire is low resistance path back to source, thus fault protection mechanism operate w. o. any delay.
In addition to that the continuous run of Earth wire on top of Tower structure discharge the electrostatic charge continuously around the TM line. This reduces the chances of lightening on conductor as well as structure. In addition lightening arrestors at terminal pole protect the substation equipment if for some reason lightening strike line conductors.
With today's technology the earth wire is replace with OPGW. Fiber optic + Ground wire.
The earth wire you see now has fiber optic cable, which carries all communication and SCADA signals. This segment of the Transmission line is most important and intelligent.
All these factors make the Earth wire essential hardware of Transmission line