I often hear that but it is no true in my experience. Maybe your run-of-the-mill flavour-of-the-month hipster web 2.0 app has a limited shelf life but most enterprise apps stay deployed for years.
But is it because the needs haven't changed a bit, or because it's good enough and it'd cost too much to replace it? Because if software was always tored out, that would only accentuate the latter problem.