some developers claim that they are gonna replace email with blockchain technology.
Blockchain is just a popular marketing buzzword. Its use is largely meaningless. You can safely ignore the marketing output of any person or business that are overly liberal with their use of buzzwords.
Wherever you see "blockchain" just substitute any of "nanotech", "quantum", "solid-state", "transistorised" or "aviation-grade" and see if it materially affects what is being said.
what guarantee they can give us that these data never gonna get decrypted 50 years from now
Generally speaking, you wouldn't choose to store data on the Bitcoin blockchain - that would be too expensive. What you might store is a hash of your data. If done correctly, that proves that your data existed at a point in time. Anyone who subsequently knows the private-keys involved can prove that they currently know the private key that was previously used. I don't think there is anything else that can be reasonably deduced or proven.
This relies on being able to check that a reasonably large number of accessible and diverse copies of the blockchain, which contain the hash, can be fully verified and agree with one another.
In this case there is no possibility† of discovering the original data from the hash. Assuming you choose your hashing algorithm well and assuming that quantum computers have not destroyed all security on the planet and plunged the world into a post-apocalyptic chaos.
† speaking in general terms. That is - the probability is perhaps not too dissimilar to that of your constituent atoms spontaneously rearranging themselves into the form of a bowl of petunias