Huh. That is such a simplistic way of viewing religion. I think you're right in a sense - that it may very well threaten religions by providing an alternative for a key reason people become religious. However, I think most religious people I know (I'm not one so I am guessing at their reasoning) would object to this, saying that there is a lot more to religion than that, and that if the person is in it only to go to heaven, they're being superficial and not really "getting" it. For that reason, I think they'd say that they do not categorize their religion as a religion because it promises to save your soul, and they'd probably also not categorize cryonics that way either.