Flow is improved by opening the valves further. Just a bigger opening doesn't necessarily make an engine breathe better. The flow path of the combustion gases, the shape of the combustion chamber, the location of the spark are all critical elements of an engines power and efficiency. They are also interlinked, a change in one feature alters the effectiveness of another feature. Also modern engines are decreasing in engine displacement and for the most part have the same or higher compression ratios. That means smaller combustion chambers and less room to open the valves into, especially important since they are always mounted on an angle to the axis of the piston.
The timing belt replacement issues are more a product of custiomers demands for better NVH (noise, vibration and harshness) belts run quieter than chains. But belt failure is less predictable than chains which is why they recommend replacing them long before they are expected to fail.