Comment by pyrale
Solar and renewables alone don't make a grid. You would also need grid-scale batteries, and the cost is not the same.
The "improvements" in power transmission is about building more lines, these lines are not going to be significantly cheaper to maintain than previous generations, and if these investment/maintenance costs are shared among less, that means more expensive electricity. Currently, in my country, electricity transport and distribution are about one third of total cost.
You have to consider that electricity will be 3x over the next 50 years (for transportation and heating). So we are currently building out a lot of extra infrastructure.
Grid scale batteries will also primarily reduce cost by offering arbitration.