Over the last decade, This American Life has been public radio’s biggest success story, building a fiercely loyal audience made up largely of listeners who do not fit public radio’s usual demographic (which is to say, most of them aren’t baby boomers.) But when host Ira Glass announced that the TAL staff were at work on a TV version of the program, longtime listeners recoiled. The TV show debuts this week on Showtime, and the radio audience is holding its breath.