Assuming that he somehow is never killed for power by his followers, is never apprehended, and even is given the ability to live forever to enact the full scope of his own personal vision, he would inexorably and inevitably be left in a vacant and ruined world all by himself.
And light will have no easier of a time making a utopia than the rulers of millennia past and future. Simply put, no good deed goes unpunished. Even if there somehow is enough to go around people will reproduce endlessly until it becomes a problem again, and due to the laws of energy in the universe we will never reach a point at which technology can effectively fulfill the needs and wants of everyone. This simply means that what one person consumes and uses is excluded from others, and simply put there is never enough to go around.
Even if everyone's desires were somehow subverted to never conflict with the happiness or well being of anyone else around them, you then immediately are faced with the second harsh reality of the human condition, being that the universe we live in is finite. Regardless of whether someone's intentions help or hurt someone else they are at the end of the day a fulfillment of what they desired to accomplish, and ultimately self pleasing and serving in nature. Good and evil are human conceptualizations, the only real "virtue" in the world occurs when someone's own desires happen to benefit, or rather, not hinder the lives of people around them. This is because, quite simply, humans will always put their own needs and wants above others in just about any and every circumstance. The very first utopia of Eden was destroyed in the lifetime of its only two inhabitants and people have been making it impossible for a sinless world to exist ever since. You see the problem with making a utopia on earth is that the nature of humanity is to subvert that exact thing. In my opinion, Light should have won, but what would follow afterwards if he was victorious might be what the writers were trying to avoid in the first place.