I think you have pointed out many of the reasons, Matt, in your posts here, and in explaining the premise for your book.
Here is what I would add.
I speak from the voice of a student and researcher, not an expert with any kind of credentials.
We have become a world-wide community. We can no longer claim that Christianity, whether fundamental or progressive, or anywhere in between has the sole Answer, or even little answers. Jesus of Nazareth was not the only - shall we say - enlightened incarnation of the intention of our Source.
Personally, I grew up in the Christian tradition and still consider myself a Christian; but I agree with the current head of the Episcopal Church US (the denomination to which I claim membership, although I am not currently active with any one congregation), Bishop Katherine Jefferts-Schori, when she said, in an interview shortly after her confirmation for that office, "We of the Episcopal church see Jesus as our pathway to the divine, but to claim that He is the only way, would be to put God in a very small box." (I'm quoting from memory, and it's been a few years now. I may not have it exactly as she said it.)
In my opinion - for what little it is worth - until it is accepted that other religions have developed from different, but equally valid perspectives, there will not be an organized institution that will appeal to the general public in today's world.
If you are interested in reading a more detailed expression of my own perspective, you can read an article I wrote last year at this location.
I like where you seem to be coming from with your work. Good luck with it.