So far, a number of companies are doing well with abstracting the protocol layer away to make focusing on building good apps easier (see mailgun.com for SMTP, context.io for IMAP, etc).
As to Threadable's long-term strategy, it goes further. :) Right now, we're building a great mailing list. As it gets further along, we plan to make it into a platform so that other app developers can use us to do all the difficult parts of managing threaded discussions, client compatibility, etc. The idea is to turn every app and website's isolated comment stream into a threaded email discussion, so people can engage using the tools they're already used to.
There's already millions upon millions of mailing lists out there, but nobody's made major changes in this space in a decade or more. We think we can add enough value that people will be happy to pay for it, and make group email collaboration more popular at the same time.
Our overall goal is the engagement and inclusivity that comes from making key information available in the way that people expect. Team collaboration tools only work well when the whole team uses them. The mailing list is just the best way to get there.
Of course there are thousands of email clients! And sure, SMTP isn't as simple as a RESTful API, but it's been a standard since 1982! Backwards compatibility is a huge win for email.
If you don't want to deal with "broken" protocols, there are services like Mailgun and Sendgrid that wrap it up in pretty API calls.
As an engineer what bugs me most in this space is the terrible email related protocols. They are broken and burdensome to deal with. I wish someone would attack that problem.
As a user what bugs me are the terrible native apps for email. I think this stems from the previous problem, few people innovate in email because SMTP and IMAP etc are terrible protocols. It explains why there aren't a thousand mail clients in the way there once were a thousand Twitter clients.
I don't know if Threadable has any plans of going beyond managing lists but those problems ^^^ really need solutions.