This is very interesting and it seems there is something to it. In the case of Buddhism, though, if the Buddha turned out to be a myth, ,
it would be very hard to continue as a Buddhist. The religion is founded, and has always been, on the belief that there was such a fellow, that he did
get enlightened, and that he taught a way of liberation based on his enlightenment. Supposing that's false, and somehow its falsity was established, it would probably be
the end of Buddhism.

In converting to Buddhism one formally takes refuge in the Buddha, the Dhamma and the Sangha. Taking refuge
in the Buddha is taking refuge in the fact that he was enlightened and that he taught what he did teach. It is accepting him as one's
teacher and inspiration. He really did it and we can do it too by following his teaching. So there is a serious
commitment to history. It is probably true that one could accept his teaching without believing he existed. Certainly his enlightenment didn't change
the metaphysics of the world or create new possibilities.But it may not follow that the details of Buddhism's founding are
inessential to its 'present commitments.' The question is, what are Buddhism's 'present commitments'?
Arguably one of them is that the Buddha existed, became enlightened and taught a way of enlightenment. And this is pretty central
in the lives of practicing Buddhists. Personally I don't think I could go on as a Buddhist if I came to disbelieve the 'founding story.'

It's not clear to me that Taoism has a founding story and I really don't know enough about Native American religion's 'founding
story' or what would count as 'demythologizing' it.

It may be there is a difference between what the Buddha taught and what Buddhism teaches. But then there is arguably a difference
between what Jesus taught and what Christianity teaches .In both cases, the latter teaches more than the former.
Also one might 'subscribe' to Buddhism, think Buddhist-type
thoughts, without BEING a Buddhist. But the same is true of Christianity. Anyhow religions can have serious historical commitments
that don't flow from the idea that the religion began in an event that changed the metaphysics of reality. (Islam may be another
example.) So your point may be made in a somewhat sharpened way.

As to Christianity, C. S. Lewis wrote that 'Christians are taking a chance.' It's hard to believe that
the historicity of the resurrection isn't part of the chance that Christians take. There is, by the way,
a powerful philosophical industry arguing that, by the best canons of historical study, the
resurrection probably happened. See William Lane Craig's debates about this on Youtube.