Uhm, what? What you describe in your suggestion isn't actually cruise control. If you've ever driven a car and used cruise control it will keep the car going at that speed, it still allows you to manually go faster but it won't drop below the speed you enabled cruise control at untill you either brake or push the clutch. What you're suggesting is an entirely different thing which would be a speed limit functionality.
And the way it currently works it simply pushes or releases the W key for you to control the car, as there's no way to properly keep velocity constant without a bunch of work.