The cost of using a blockchain at the scale of one of today's big payment processors is still unknown. The current amount of transactions per second in something like Bitcoin is ridiculous compared to Visa or even any big online retailer. Some say that it could scale to that level but it remains to be seen.
Regarding email it's unfortunately a rather bad example. While I personally host my own email server on a dedicated host I seem to be among a rather small minority. Every time this subject comes up on HN people say that they had a lot of trouble setting up their own email server because they kept being flagged as spam by the big hosts like gmail and friends.
Email is clearly undergoing a rather massive centralization around a few big providers offering an ad-sponsored service.
There are also obvious advantages to centralizing email: when you have many users you can de-duplicate content more aggressively, you can catch spam much more effectively etc... Overall it is more efficient, privacy and independence notwithstanding.
There's also an other important difference between email and blockchains: even if you consider email a decentralized service not all nodes (SMTP servers) need to store the full state and history of all emails sent in the entire world. They just pass emails along and only keep those addressed to a user of this particular server. It's similar to other services like IRC or bittorrent for instance.
Full Bitcoin nodes need to store the full state of the blockchain to function. You can optimize it a bit if you're willing to lose some functionality (discard spent outputs for instance) but not everybody can do that or the system fails and that's still a massive amount of data to keep around forever.
Email is (potentially) decentralized but also distributed. Blockchain is decentralized but every node contains the full state of the system. That's a very big difference when it comes to scalability.
But you would first need to develop a system where it would be common to send millions of dollars to untrusted parties at high frequency before you would ever use these zero-fee cryptos.
Email surely isn't central, and it seems to be pretty cheap for consumers. Do you think we should centralize email to make it better?