I never said resistance will change. (Only partially true though because heat plays a big role on resistance)
Ohms law states that voltage and current are proportional.
I see what you are saying.
With your power line theory we can go 10kv over the line which lets say is 100 ohm now let's check current flow.
V = IR
10k = I x 100
I = 100 amp
Let's now reduce the voltage at the same resistance.
5kv = I x 100
I = 50 amp
Current is voltage flow. It will flow directly proportional to that of voltage if resistance does not change.
In order to calculate power with no other factors changing which you have done, you have to go back to the first step and see if any factors have changed ie resistance would most probably have to change in that factor.
I get a sense that some of this might be misinterpreted as "having a go" but if in person this would turn into a great discussion.