My understanding is that used to be how most text messaging was done pre-smartphone in Japan.
Currently the only similar thing I'm aware of is https://delta.chat/en/, though I believe it does all of its networking over email, rather than only using it as a fallback.
I wonder what the pitfalls of using email this way are; it seems like a great way to get a free backend and growth-hack a chat app, so there must be some reason it's not more common.
It's the idealist's solution because it benefits the user. Companies typically want to use phone numbers because they're more expensive for users to maintain separate identities with, which helps when you want to track them. Moreover, companies want lock-in to their own network effect, not a federated network that anybody else can permissionlessly join.
It's the sort of thing you get when somebody builds it as a hobby project, or a skunkworks project escapes from a large corporation and is already open source by the time the MBAs get their hands on it. Or, in the old days, DARPA funding.
Currently the only similar thing I'm aware of is https://delta.chat/en/, though I believe it does all of its networking over email, rather than only using it as a fallback.
I wonder what the pitfalls of using email this way are; it seems like a great way to get a free backend and growth-hack a chat app, so there must be some reason it's not more common.