And now I'm curious where you live that most people don't accelerate past 70 mph...I think most people in this thread would agree that the issue is nowhere near as dramatic at lower levels.
However, speaking on behalf of those who do regularly play campaigns that approach 20th level, it would be nice if the fighter managed to maintain reasonable effectiveness alongside the wizard.
Even if you, yourself, never touch those levels. I don't find the idea that it's okay for high levels not to work well because not many people go to those levels, to be an especially compelling argument. Most people don't take their car over 70 MPH but people would be quite upset, I think, if they did accelerate past 70 and the engines consistently dropped out.
I think there is a difference between asking for 80 mph, 90mph, 120mph, and keeping up with a helicopter.