Where do religions go when they die?

Kandy, Sri Lanka
I heard Philip Jenkins interviewed several weeks ago on CBC about his new book, The Lost History of Christianity, in which he discussed the early spread of Christianity into Africa and Asia, followed by its later disappearance. In another interview with beliefnet he responds to the theological dilemmas presented when a religion anticipates its ultimate victory long after it has climaxed:
There is a major theological issue that nobody addresses, the theology of extinction. How do Christians explain the death of their religion in a particular time and place? Is that really part of God’s plan? Or maybe our time scale is just too short, and one day we will realize why this had to happen. But as I say, nobody is really discussing these questions.
I can understand how it must be troubling for Christians to contemplate such questions, but they need to asked and understood. Virtually all of the world religions we know today emerged during a very short period of time when civilizations grew and expanded — not too far in the past, relatively speaking (less than 4,000 years ago). In the previous 150,000 years of human existence, surely there were religions that were born and gradually retreated as social evolution outgrew them. Baha’u'llah’s writings offer one way out of the theological dilemma presented by Jenkins:
These principles and laws, these firmly-established and mighty systems, have proceeded from one Source and are the rays of one Light. That they differ one from another is to be attributed to the varying requirements of the ages in which they were promulgated.