NBC's massively successful drama will have its series finale next week. It brought in the ratings and the bucks, but its real legacy may be bringing lifelike authenticity to prime time. With its technical innovations and reliance on realism, "ER" changed dramatic television. And when NBC pulls the plug on the show Thursday after 15 seasons, "ER" will leave behind a splintered prime-time landscape as the networks struggle to compete in a digital world.